fix(terminal): address re-review findings on the pane-buffer rework

Follow-up to the PR #112 re-review (all six prior blockers were already
resolved; these are new issues the rework introduced):

- app.js: define the missing `_scheduleTerminalRepaint()` helper. It was
  called from both WebGL-fallback paths (onContextLoss + long-task trip)
  but defined nowhere, so each fallback threw `TypeError` and lost the
  post-fallback repaint, leaving a stale/blank terminal. Implemented as an
  rAF-debounced full refresh (matches the old inline `terminal.refresh`).
- app.js: clear terminal load-state on the two post-write stale-select
  early-returns (cached-buffer + rewrite branches), matching the other
  four checks. Switching away from a mid-loading tab no longer leaks a
  permanent `.tab-loading` spinner / `aria-busy=true`.
- terminal-ui.js + app.js: gate the post-resize TUI-redraw settle on an
  actual dimension change. `sendResize` now returns whether dims changed;
  a same-size tab switch sends no SIGWINCH, so the wait is skipped instead
  of charging a flat tax on every non-shell switch. Literal hoisted to
  `TUI_REDRAW_SETTLE_MS`.
- tmux-manager.ts: `resizeWindow()` uses a non-blocking `exec` instead of
  `execSync` so the interactive WS/HTTP resize path can't stall the
  Fastify event loop on a slow/hung tmux. Sole caller already fire-and-
  forgets the result; test updated to assert the async dispatch.

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

src/web/public/constants.js

@@ -57,6 +57,7 @@ const TERMINAL_CHUNK_SIZE = 32 * 1024;      // 32KB chunks for terminal buffer l
 const TERMINAL_TAIL_SIZE = 1024 * 1024;     // 1MB tail for initial load (more scrollback on tab switch)
 const SYNC_WAIT_TIMEOUT_MS = 50;            // Wait timeout for terminal sync
 const STATS_POLLING_INTERVAL_MS = 2000;     // System stats polling
+const TUI_REDRAW_SETTLE_MS = 400;           // Grace for a TUI to redraw after a real resize, before fetching its buffer
 
 // Z-index base values for layered floating windows
 const ZINDEX_SUBAGENT_BASE = 1000;