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/SearchBar.tsx

@@ -61,18 +61,20 @@ export function SearchBar({
   return (
     <div
       style={{
+        // Anchored to the focused panel's card (position:relative). Sits over
+        // the 30px header so it's obvious which panel is being searched.
         position: "absolute",
-        top: 8,
-        right: 16,
+        top: 3,
+        right: 6,
         zIndex: 50,
         display: "flex",
         alignItems: "center",
-        gap: 6,
-        height: 32,
-        padding: "0 8px",
+        gap: 4,
+        height: 24,
+        padding: "0 6px",
         background: COLORS.bgElevated,
         border: `1px solid ${COLORS.borderStrong}`,
-        borderRadius: 8,
+        borderRadius: 6,
         boxShadow: "0 4px 16px rgba(0,0,0,0.4)",
       }}
     >
@@ -100,7 +102,7 @@ export function SearchBar({
         }}
         placeholder="Search scrollback"
         style={{
-          width: 200,
+          width: 150,
           background: "transparent",
           border: "none",
           outline: "none",