fix: search field never narrower than its own placeholder

The field could shrink to 160px and clip 'Search videos and channels'.
Its floor is now 17rem, wide enough for the full placeholder, with a
roomier max so it uses spare width.

The window minimum rises to 1260 to keep the control row on one line at
that floor, and the default size moves to 1320x820 — it was below the
new minimum, so every launch was being silently clamped.

src-tauri/tauri.conf.json

@@ -13,9 +13,9 @@
     "windows": [
       {
         "title": "FlightTube",
-        "width": 1180,
-        "height": 780,
-        "minWidth": 1140,
+        "width": 1320,
+        "height": 820,
+        "minWidth": 1260,
         "minHeight": 620,
         "center": true,
         "titleBarStyle": "Overlay",