(ui): replace native confirm/alert with the designed dialog everywhere, and ban them #585

Decision (maintainer, 12/10): every confirmation and message in the renderer goes through the app's designed dialog, the same one as the close-session confirmation (showChoiceDialog, public/choice-dialog.js). Native confirm(), alert() and prompt() are banned. A lint rule enforces the ban, and a small substitution module is the only entry point.

Why

• The native dialogs look out of place: the resume, attach and live-elsewhere guards, deletes and save failures all use them.
• They block the renderer.
• Playwright auto-dismisses them, so e2e journeys cannot drive them. QA run 2 hit this.

Inventory on be8566c (grep -nE "\b(window\.)?(confirm|alert|prompt)\(" public/*.js)

• resume-guard.js:33,42,56,62,68,72: scheduled task already running, live elsewhere, continuation choice. Injected as confirm from app.js:307,1519,1551.
• app.js:1054: delete plan confirm.
• agents-view.js:421: delete background session.
• sidebar.js:1169,1236,1238,1241: archive messages. sidebar.js:1313,1316: remap. sidebar.js:1343,1360: hide or delete worktree.
• settings-panel.js:715: hide project.
• viewer-panel.js:283,290,293: delete file. viewer-panel.js:473: overwrite a file changed on disk. viewer-panel.js:594: discard unsaved edits.
• file-panel.js:438,448,451: overwrite and save failure. file-panel.js:1456: discard unsaved edits. This one is sync, inside the unsaved guard ((restore): save the open sessions when the app closes #479/(window): ask before a window close ends the app #478), so take care.
• activity-trace-panel.js:70,120,133: read, delete, delete failure.

Expected

1. A module, for example public/app-dialogs.js, exposing async appConfirm({ title, message, confirmLabel, danger }) and appAlert({ title, message }), built on showChoiceDialog.
   • It queues instead of resolving null when a dialog is already open. showChoiceDialog currently returns null at once in that case, which would read as Cancel.
   • Focus follows the (a11y): New Session dialog opened from the keyboard leaves focus on the page body #583 rule: on open, the safe button for a destructive confirm, or the first control; on close, back to the opener.
2. Every call site above migrated. Sync callers become async. This matters most on the save and unsaved-edit paths, and wherever an injected confirm is used: resume-guard.js takes an async confirm.
3. ESLint no-restricted-globals / no-restricted-properties for confirm, alert and prompt (bare and window.) over public/**, with a message pointing to the module.
4. Tests:
   • The module: queueing, focus, Escape = cancel.
   • Each migrated guard keeps its semantics.
   • One e2e journey drives a formerly native confirm, for example Delete on a background session, or a resume guard.
5. Main-process native dialogs (dialog.showMessageBox, for example the "Close Switchboard?" quit question) are out of scope. List them in the PR as a follow-up.