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/src/commands/breadclip.rs

@@ -0,0 +1,8 @@
+//! breadclip has no config file to edit — this panel exists purely to make
+//! its background daemon (breadclipd, via `service.rs`) and its
+//! on-demand popup visible/controllable from Settings.
+
+#[tauri::command]
+pub fn open_breadclip() {
+    let _ = std::process::Command::new("breadclip").spawn();
+}