browser: Reduce spurious rendering and lock contention

[Yabuku-xD]

Summary

Four targeted, low-risk improvements to the browser crate's rendering pipeline — no new dependencies, no behavior changes, purely less wasted work:

• Gate FrameReady on suspension (tab.rs): Skip emitting TabEvent::FrameReady for suspended tabs, matching the existing is_suspended guard on all other events.
• Gate FrameReady notify on surface visibility (browser_view.rs): Only call cx.notify() from FrameReady when surface_state == Visible. In-flight frames queued just before a mode switch no longer trigger layout work on the hidden browser view.
• Split RenderState Mutex (render_handler.rs, tab.rs, client.rs, life_span_handler.rs): Extract current_frame: Option<CVPixelBuffer> into its own Arc<Mutex<...>>. CEF's view_rect/screen_info geometry callbacks no longer share a lock with the 60fps on_accelerated_paint write path. Geometry queries and frame buffer writes now operate on independent locks — no contention between them.
• Compute pinned_count once per render (tab_strip.rs, browser_view.rs): Both render_tab_strip and render_sidebar previously performed independent O(n) scans over all tabs to count pinned ones. A pinned_tab_count() helper is now called once in render() and passed as a parameter to both methods.

Test plan

• cargo check -p browser passes with no errors
• cargo check -p workspace -p browser passes
• Mode switching (browser → editor → terminal → browser) remains smooth with no stale frames
• Tab suspension/resume works correctly with no blank-frame artifacts

Release Notes:

• N/A