fix(app): working scrollback search + stop prompt duplication on focus

Search fixes:
- TerminalView sets allowProposedApi (the search addon's match decorations
  use registerMarker/registerDecoration); without it findNext threw before
  firing results, so the counter was stuck at 0/0.
- The search bar now renders inside the panel it targets (in the header)
  instead of a global top-right overlay, so it's obvious which panel is
  searched.
- Search is anchored to the panel it was opened on (searchSurfaceId) and no
  longer follows focus — opening it in one panel and switching away no longer
  shows it open elsewhere.

Prompt duplication:
- The focus border was 1px when unfocused, 2px when focused; with border-box
  that resized the content on every focus switch, firing ResizeObserver -> fit
  -> PTY SIGWINCH and making zsh/powerlevel10k reprint its prompt. The border
  is now a constant 2px, color-only on focus.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/TerminalView.tsx

@@ -14,7 +14,10 @@ export function TerminalView({ surfaceId }: { surfaceId: string }) {
 
   useEffect(() => {
     if (!ref.current) return;
-    const term = new Terminal({ fontFamily: "'JetBrains Mono Variable', 'JetBrains Mono', monospace", fontSize: 13, convertEol: false, scrollback: 10000 });
+    // allowProposedApi is required by the search addon: its match decorations
+    // call registerMarker/registerDecoration (proposed API). Without it findNext
+    // throws and the scrollback search counter never updates.
+    const term = new Terminal({ fontFamily: "'JetBrains Mono Variable', 'JetBrains Mono', monospace", fontSize: 13, convertEol: false, scrollback: 10000, allowProposedApi: true });
     try {
       term.loadAddon(new WebglAddon());
     } catch {