feat: add Vietnamese localization to GUI

Introduces a two-language (EN/VI) i18n system with hot-swap support.
All ~160 user-facing strings are centralised in gui/locales.py; views
retranslate in-place on language switch without restarting the app.

- gui/locales.py: new file — _EN/_VI dicts, t() lookup, set_language(),
  get_language(); assert guard ensures EN/VI key parity
- gui/app.py: switch_language(), _apply_startup_language(),
  _save_language_pref(), _rebuild_nav_labels(); language stored in
  config.json under ui.language; pipeline step headers and run_tool
  status lines translated
- gui/views/settings.py: Language dropdown card (English / Tiếng Việt)
- gui/views/dashboard.py: all strings via t(); static header widgets
  stored and retranslated in refresh()
- gui/views/mods.py: all strings via t(); _STATUS dict built at call
  time so server status labels update on language switch
- gui/views/tools.py: all strings via _translatable registry; tab names
  and segmented-button values kept in English (CTkTabview constraint)
- gui/views/logs.py: title + Copy/Clear buttons stored, retranslated
- gui/wizard.py: all 3 pages fully translated
- docs/huong-dan-su-dung.md: full Vietnamese user guide
- CLAUDE.md: documents localization architecture and constraints

gui/app.py

@@ -15,6 +15,7 @@ from gui._constants import (
     SIDEBAR_W, APP_TITLE, PROJECT_ROOT, SELECTION_FILE,
 )
 from gui._io import _QueueWriter
+from gui.locales import t
 from gui.wizard import SetupWizard
 from gui.views.base import BaseView
 
@@ -62,6 +63,7 @@ class ArmaModManagerApp(ctk.CTk):
         else:
             self._load_config()
 
+        self._apply_startup_language()
         self._build_layout()
         self._poll_log()
 
@@ -106,21 +108,31 @@ class ArmaModManagerApp(ctk.CTk):
         """Thread-safe: enqueue text for the Logs panel."""
         self._log_q.put(text)
 
+    def switch_language(self, lang: str) -> None:
+        """Switch the UI language and refresh all cached views."""
+        from gui import locales
+        locales.set_language(lang)
+        self._save_language_pref(lang)
+        self._rebuild_nav_labels()
+        for view in self._view_cache.values():
+            view.refresh()
+        if self._active_name:
+            self.navigate_to(self._active_name)
+
     def run_pipeline(self, selected_names: set[str]) -> None:
         """Start the background pipeline for the given preset filenames."""
         if self._pipeline_running:
             return
         if len(selected_names) < 2:
             messagebox.showwarning(
-                "Not enough presets selected",
-                "Please select at least 2 preset files to compare.\n\n"
-                "Use the checkboxes on the Dashboard to choose which presets to use.",
+                t("app.dlg_presets_title"),
+                t("app.dlg_presets_body"),
             )
             return
 
         cfg = self._cfg
         if not cfg:
-            messagebox.showwarning("Setup required", "Please complete Setup first.")
+            messagebox.showwarning(t("app.dlg_setup_title"), t("app.dlg_setup_body"))
             return
 
         self._pipeline_running = True
@@ -137,7 +149,7 @@ class ArmaModManagerApp(ctk.CTk):
                 from arma_modlist_tools.compare import compare_presets
 
                 # Step 1 — Parse selected presets
-                _hdr("Step 1 / 4", "Parse presets")
+                _hdr("Step 1 / 4", t("pipeline.step1_name"))
                 cfg.modlist_json.mkdir(exist_ok=True)
                 presets = []
                 for fp in sorted(cfg.modlist_html.glob("*.html")):
@@ -154,7 +166,7 @@ class ArmaModManagerApp(ctk.CTk):
                     presets.append(preset)
 
                 # Step 2 — Compare
-                _hdr("Step 2 / 4", "Compare presets")
+                _hdr("Step 2 / 4", t("pipeline.step2_name"))
                 result = compare_presets(*presets)
                 cfg.comparison.write_text(
                     json.dumps(result, indent=2, ensure_ascii=False),
@@ -166,11 +178,11 @@ class ArmaModManagerApp(ctk.CTk):
                       f"Unique: {total_unique}")
 
                 # Step 3 — Fetch
-                _hdr("Step 3 / 4", "Download mods")
+                _hdr("Step 3 / 4", t("pipeline.step3_name"))
                 step_fetch(cfg)
 
                 # Step 4 — Link
-                _hdr("Step 4 / 4", "Link mods")
+                _hdr("Step 4 / 4", t("pipeline.step4_name"))
                 groups = (
                     sorted(p.name for p in cfg.downloads.iterdir() if p.is_dir())
                     if cfg.downloads.is_dir() else []
@@ -212,11 +224,13 @@ class ArmaModManagerApp(ctk.CTk):
                     self.post_log(line)
                 proc.wait()
                 ok = proc.returncode == 0
-                self.post_log(
-                    f"\n{'✓  Done' if ok else f'✗  Exited with code {proc.returncode}'}.\n"
+                done_msg = (
+                    t("app.tool_done") if ok
+                    else t("app.tool_exit_code", code=proc.returncode)
                 )
+                self.post_log(f"\n{done_msg}.\n")
             except Exception as e:
-                self.post_log(f"\n✗  Failed to start {script}: {e}\n")
+                self.post_log(f"\n{t('app.tool_failed', script=script, e=e)}\n")
 
         threading.Thread(target=worker, daemon=True).start()
 
@@ -241,6 +255,38 @@ class ArmaModManagerApp(ctk.CTk):
     def open_wizard(self) -> None:
         SetupWizard(self, on_complete=self._after_wizard)
 
+    # =========================================================================
+    # Private — language
+    # =========================================================================
+
+    def _apply_startup_language(self) -> None:
+        """Read language preference from config.json and activate it."""
+        from gui import locales
+        lang = "en"
+        cfg_path = PROJECT_ROOT / "config.json"
+        if cfg_path.exists():
+            try:
+                raw = json.loads(cfg_path.read_text(encoding="utf-8"))
+                lang = raw.get("ui", {}).get("language", "en")
+            except Exception:
+                pass
+        locales.set_language(lang)
+
+    def _save_language_pref(self, lang: str) -> None:
+        """Persist language preference into the 'ui' key of config.json."""
+        cfg_path = PROJECT_ROOT / "config.json"
+        try:
+            raw = json.loads(cfg_path.read_text(encoding="utf-8"))
+            raw.setdefault("ui", {})["language"] = lang
+            cfg_path.write_text(json.dumps(raw, indent=2), encoding="utf-8")
+        except Exception:
+            pass  # non-fatal — language preference simply resets next run
+
+    def _rebuild_nav_labels(self) -> None:
+        """Retranslate the sidebar navigation button labels."""
+        for name, btn in self._nav_btns.items():
+            btn.configure(text=t(f"nav.{name.lower()}"))
+
     # =========================================================================
     # Private — layout
     # =========================================================================
@@ -264,7 +310,7 @@ class ArmaModManagerApp(ctk.CTk):
         self._nav_btns: dict[str, ctk.CTkButton] = {}
         for i, name in enumerate(_VIEW_NAMES, start=1):
             b = ctk.CTkButton(
-                sb, text=name, width=SIDEBAR_W - 24,
+                sb, text=t(f"nav.{name.lower()}"), width=SIDEBAR_W - 24,
                 anchor="w", command=lambda n=name: self.navigate_to(n),
                 fg_color="transparent",
                 hover_color=("gray80", "gray30"),