feat: fix mods tab, add client/server split, and scaffold server dirs

Mods tab bug fixes:
- mod_manager: fix wrong kwargs in set_enabled_mods, fix scan dir to use
  mods/ subdir instead of server root, migrate old string-list format to
  dict format on read
- service: replace dead server_mods SQL JOIN with get_enabled_mods()
  call through the mod_manager capability; pass is_server_mod to
  build_mod_args
- mods_router: accept list[EnabledModEntry] objects (name + is_server_mod)
  instead of bare strings

Client/server mod split:
- Mods now stored as list[{"name": str, "is_server_mod": bool}]; old
  string-list format auto-migrated on read
- is_server_mod=true routes to -serverMod= arg; false to -mod= arg
- ModList UI: amber Client/Server badge in selected pane; toggle button
  in split-pane selector

Directory scaffold:
- process_config: adds "mods" to dir layout; provides get_dir_readme()
  with per-directory README.txt content
- file_utils: ensure_server_dirs() gains readme_provider kwarg; writes
  README.txt idempotently if absent
- service.create_server: passes readme_provider via hasattr probe
- main.py startup: backfills all existing servers with correct subdirs
  and README files (idempotent)

Docs: API.md and FRONTEND.md updated for new mod schema and types
Test __init__.py files added for pytest discovery

frontend/src/components/servers/ModList.tsx

@@ -1,5 +1,5 @@
 import { useState, useEffect } from "react";
-import { Save } from "lucide-react";
+import { Save, Server } from "lucide-react";
 
 import { useServerMods, useSetEnabledMods } from "@/hooks/useServerDetail";
 import type { Mod } from "@/hooks/useServerDetail";
@@ -38,14 +38,25 @@ export function ModList({ serverId }: ModListProps) {
     setAvailable((prev) => [...prev, { ...mod, enabled: false }].sort((a, b) => a.name.localeCompare(b.name)));
   };
 
+  const toggleServerMod = (modName: string) => {
+    setSelected((prev) =>
+      prev.map((m) => m.name === modName ? { ...m, is_server_mod: !m.is_server_mod } : m),
+    );
+  };
+
+  const _selectedKey = (mods: Mod[]) =>
+    mods.map((m) => `${m.name}:${m.is_server_mod}`).sort().join(",");
+
   const hasChanges = modsData !== undefined && (
-    selected.map((m) => m.name).sort().join(",") !==
-    (modsData.mods.filter((m) => m.enabled).map((m) => m.name).sort().join(","))
+    _selectedKey(selected) !==
+    _selectedKey(modsData.mods.filter((m) => m.enabled))
   );
 
   const handleApply = async () => {
     try {
-      await setEnabledMods.mutateAsync(selected.map((m) => m.name));
+      await setEnabledMods.mutateAsync(
+        selected.map((m) => ({ name: m.name, is_server_mod: m.is_server_mod })),
+      );
       addNotification({ type: "success", message: `${selected.length} mod(s) enabled. Server restart required.` });
     } catch (err) {
       logger.error("ModList", "Failed to apply mods: %s", err);
@@ -144,6 +155,7 @@ export function ModList({ serverId }: ModListProps) {
                   mod={mod}
                   actionLabel="←"
                   onAction={isAdmin ? () => moveToAvailable(mod) : undefined}
+                  onToggleServerMod={isAdmin ? () => toggleServerMod(mod.name) : undefined}
                   selected
                 />
               ))
@@ -159,17 +171,17 @@ function ModRow({
   mod,
   actionLabel,
   onAction,
+  onToggleServerMod,
   selected = false,
 }: {
   mod: Mod;
   actionLabel: string;
   onAction?: () => void;
+  onToggleServerMod?: () => void;
   selected?: boolean;
 }) {
   return (
-    <div
-      className="flex items-center gap-2 px-2 py-1.5 rounded-lg bg-surface-recessed shadow-neu-recessed"
-    >
+    <div className="flex items-center gap-2 px-2 py-1.5 rounded-lg bg-surface-recessed shadow-neu-recessed">
       <div className="flex-1 min-w-0">
         <p className="text-text-primary text-sm font-medium truncate">
           {mod.display_name ?? mod.name}
@@ -186,6 +198,21 @@ function ModRow({
           <span className="text-text-muted text-xs">{formatSize(mod.size_bytes)}</span>
         </div>
       </div>
+      {selected && onToggleServerMod && (
+        <button
+          type="button"
+          onClick={onToggleServerMod}
+          title={mod.is_server_mod ? "Server-only mod (-serverMod). Click to switch to client mod (-mod)" : "Client mod (-mod). Click to switch to server-only (-serverMod)"}
+          className={`flex items-center gap-1 text-xs px-1.5 py-0.5 rounded shrink-0 transition-colors ${
+            mod.is_server_mod
+              ? "bg-amber-500/20 text-amber-400"
+              : "bg-surface-raised text-text-muted hover:text-text-secondary"
+          }`}
+        >
+          <Server size={10} />
+          {mod.is_server_mod ? "Server" : "Client"}
+        </button>
+      )}
       {onAction && (
         <button
           type="button"