fix(web): reliable terminal copy with fallback, feedback, and shift-drag hint

[Bdandc]

Problem

Copy/paste from the dashboard terminal (xterm.js + tmux) was unreliable, with zero feedback when it failed:

1. Silent failures — every copy path ended in navigator.clipboard.writeText(...).catch(() => {}). navigator.clipboard is undefined on non-secure origins (dashboard opened via LAN IP instead of localhost) and can be permission-denied, so copies silently no-oped.
2. Mouse capture — agent TUIs (Claude Code) enable mouse reporting in tmux, so xterm.js forwards drags to the app instead of selecting text. Shift+drag forces local selection, but nothing told the user.

Changes

• writeClipboardText() layered helper (terminal-clipboard.ts): navigator.clipboard.writeText → document.execCommand('copy') via a temporary sr-only textarea (works on non-secure origins, restores focus afterwards) → resolves false so the failure is surfaced instead of swallowed. Used by both terminal copy paths: the OSC 52 handler and the Cmd+C / Ctrl+Shift+C key handler (these are the only terminal copy paths — verified by sweep).
• Copy feedback: registerClipboardHandlers reports every copy outcome via a new optional onCopyResult callback. DirectTerminal flashes a small transient Copied / Copy failed pill (role="status") over the terminal for 2s. Design tokens + Tailwind classes only — no inline styles, no new libraries (C-01, C-02, C-07).
• Discoverability hint: useXtermTerminal detects active mouse reporting via terminal.modes.mouseTrackingMode (checked in the parsed-write callback, setState only on transitions). While active, TerminalControls shows a subtle ⇧+drag to select hint with an explanatory tooltip, in both the chrome bar and the chromeless floating controls. It disappears when the TUI releases the mouse.

The optional copy-on-select toggle was deliberately left out — reliable selection-end detection on top of onSelectionChange isn't low-risk, and the core fix doesn't depend on it. Happy to follow up if wanted.

Invariants preserved

• The terminal-creation effect's dependency list is unchanged — the copy callback flows through a ref (onCopyResultRef) so a changing identity can't tear down/recreate the terminal or WebSocket.
• Selection-preserving write buffering, follow-output scrolling, and the XDA/OSC 52 wiring are untouched apart from passing the write-parsed callback.

Tests

• terminal-clipboard.test.ts extended (15 new tests): writeClipboardText matrix — clipboard present / absent / rejected, execCommand ok / false / throws / missing, textarea cleanup — and onCopyResult reporting for OSC 52 success, decode failure, fallback success/failure, and Cmd+C.
• New DirectTerminal.copy-feedback.test.tsx: toast show/auto-dismiss/timer-reset, hint visible while mouse reporting active (chrome + chromeless), hidden otherwise.

Verification

• pnpm --filter @aoagents/ao-web typecheck ✅
• pnpm --filter @aoagents/ao-web test ✅ (92 files, 1050 passed, 5 skipped)
• pnpm lint ✅ (0 errors; remaining warnings pre-existing)
• Prettier check on all touched files ✅

🤖 Generated with Claude Code