Migrate bos-settings from GTK4 to Tauri + Svelte 5

Ports every remaining view (Keybinds, the last one — dual Flat/MultiLayout
schema detection, per-action dedicated fields with a raw-JSON fallback for
anything not modeled) and removes the now-fully-dead GTK4 crate (root
Cargo.toml + src/), leaving src-tauri/ as the single Rust binary crate.

Also folds in the UX pass done alongside the migration: responsive
multi-column layout instead of a single centered column, chip/dropdown
pickers replacing free-typed fields (Wi-Fi networks, Bar style, keyboard
layout, tag lists), consistent pill-switch toggles and boxed-list card
styling across every page, and a sidebar scroll-chaining fix.

src-tauri/tauri.conf.json

@@ -0,0 +1,41 @@
+{
+  "$schema": "https://schema.tauri.app/config/2",
+  "productName": "BOS Settings",
+  "version": "0.8.0",
+  "identifier": "dev.breadway.bos-settings",
+  "build": {
+    "beforeDevCommand": "npm --prefix frontend run dev",
+    "devUrl": "http://localhost:1420",
+    "beforeBuildCommand": "npm --prefix frontend run build",
+    "frontendDist": "../frontend/build"
+  },
+  "app": {
+    "windows": [
+      {
+        "title": "BOS Settings",
+        "width": 960,
+        "height": 640,
+        "decorations": false,
+        "backgroundColor": "#0c0c0c"
+      }
+    ],
+    "security": {
+      "csp": null,
+      "assetProtocol": {
+        "enable": true,
+        "scope": ["$HOME/Pictures/Backgrounds/**"]
+      }
+    }
+  },
+  "bundle": {
+    "active": true,
+    "targets": "all",
+    "icon": [
+      "icons/32x32.png",
+      "icons/128x128.png",
+      "icons/128x128@2x.png",
+      "icons/icon.icns",
+      "icons/icon.ico"
+    ]
+  }
+}