feat: Make app fully portable

- Add tauri-plugin-store for portable data storage
- Implement portable data directory (TypoGenie-Data/ next to EXE)
- Configure Rust backend to use EXE-relative paths
- Add store permissions for persistent settings
- Update README with portable badges and documentation
- Document how to build and use the portable EXE
- Zero registry, zero AppData, fully self-contained

src-tauri/capabilities/default.json

@@ -1,7 +1,7 @@
 {
   "$schema": "../gen/schemas/desktop-schema.json",
   "identifier": "default",
-  "description": "Default capabilities for TypoGenie",
+  "description": "Default capabilities for TypoGenie portable app",
   "windows": [
     "main"
   ],
@@ -19,9 +19,22 @@
     "fs:allow-rename",
     "fs:allow-exists",
     "fs:allow-mkdir",
+    "fs:allow-applocaldata-read",
+    "fs:allow-applocaldata-write",
+    "store:default",
+    "store:allow-get",
+    "store:allow-set",
+    "store:allow-save",
+    "store:allow-load",
     {
       "identifier": "fs:scope",
       "allow": [
+        {
+          "path": "$EXE/**"
+        },
+        {
+          "path": "$EXE/../**"
+        },
         {
           "path": "$HOME"
         },