Add Sync now button that uploads a bookmarks snapshot

Wire `SyncEngine::upload_bytes` to a Settings → Sync button:

- `BrowserCore::build_sync_snapshot_bytes` serialises the user's
  bookmarks on the UI thread (cheap, synchronous).
- `ElyShell::trigger_cloud_sync_upload` resolves the active
  profile data dir, spawns a dedicated `ely-sync-upload` thread,
  and lets the engine run the blocking HTTP round-trip there so
  the GPUI render loop never stalls on the network — the same
  invariant the Servo IPC worker enforces.
- Outcomes go through `tracing` on the `ely::sync` target. Users
  drop a Better Auth bearer token into
  `<profile_data>/sync/bearer.token` to opt in; without one, the
  engine reports `SignedOut` and the click is a no-op.

The Better Auth handshake + device-approval UX still need their
own UI passes; this lands the data-plane plumbing so those pieces
slot in without re-architecting the snapshot path.

crates/ely_app/src/shell/internal_pages/sync.rs

@@ -136,19 +136,40 @@ fn render_metric(label: &'static str, value: usize, color: u32) -> AnyElement {
 
 fn render_reset_button(cx: &mut Context<ElyShell>) -> AnyElement {
     div()
-        .id(SharedString::from("sync-reset"))
-        .px(px(12.0))
-        .py(px(7.0))
-        .rounded(px(8.0))
-        .bg(rgba(BUTTON_BG))
-        .text_size(px(12.0))
-        .font_weight(FontWeight(500.0))
-        .text_color(rgb(colors::INK_2))
-        .cursor_pointer()
-        .hover(|style| style.bg(rgba(BUTTON_BG_HOVER)))
-        .active(|style| style.opacity(0.85))
-        .on_click(cx.listener(|shell, _, _, cx| shell.reset_sync_settings(cx)))
-        .child("Reset to defaults")
+        .flex()
+        .gap(px(8.0))
+        .child(
+            div()
+                .id(SharedString::from("sync-upload"))
+                .px(px(12.0))
+                .py(px(7.0))
+                .rounded(px(8.0))
+                .bg(rgba(colors::ACCENT))
+                .text_size(px(12.0))
+                .font_weight(FontWeight(500.0))
+                .text_color(rgb(0xfff5e6))
+                .cursor_pointer()
+                .hover(|style| style.opacity(0.92))
+                .active(|style| style.opacity(0.78))
+                .on_click(cx.listener(|shell, _, _, cx| shell.trigger_cloud_sync_upload(cx)))
+                .child("Sync now"),
+        )
+        .child(
+            div()
+                .id(SharedString::from("sync-reset"))
+                .px(px(12.0))
+                .py(px(7.0))
+                .rounded(px(8.0))
+                .bg(rgba(BUTTON_BG))
+                .text_size(px(12.0))
+                .font_weight(FontWeight(500.0))
+                .text_color(rgb(colors::INK_2))
+                .cursor_pointer()
+                .hover(|style| style.bg(rgba(BUTTON_BG_HOVER)))
+                .active(|style| style.opacity(0.85))
+                .on_click(cx.listener(|shell, _, _, cx| shell.reset_sync_settings(cx)))
+                .child("Reset to defaults"),
+        )
         .into_any_element()
 }