feat(desktop): rebrand T3 Code → ClayCode in user-visible surfaces

[cschubiner]

Summary

Restores the ClayCode brand on user-visible desktop-app surfaces.
Internal IDs (bundle id, user-data dir, WM class, dev-flag, worktree
prefix, legacy-migration paths) are intentionally preserved so existing
installs upgrade cleanly without losing data or duplicating dock icons.

Changes

• `apps/desktop/src/appBranding.ts` — `APP_BASE_NAME` flips to `ClayCode`; all
  helper-derived display names follow.
• `apps/desktop/src/appBranding.test.ts` — fixture update.
• `apps/desktop/scripts/electron-launcher.mjs` — dev/alpha display names + leading comment.
• `apps/desktop/src/main.ts` — replaced two hardcoded `T3 Code` strings (startup error
  dialog and "up to date" dialog) with `desktopAppBranding.baseName` references so
  future renames only need a single edit in `appBranding.ts`.
• `apps/desktop/package.json` — `productName: ClayCode (Alpha)` for electron-builder
  artifact naming.

Intentionally NOT renamed

• `com.t3tools.t3code` / `com.t3tools.t3code.dev` bundle IDs
• `USER_DATA_DIR_NAME` (`t3code`/`t3code-dev`)
• `LINUX_DESKTOP_ENTRY_NAME`, `LINUX_WM_CLASS`
• `LEGACY_USER_DATA_DIR_NAME` (`T3 Code (Dev|Alpha)` — required for legacy migration)
• `--t3code-dev-root` CLI flag
• `WORKTREE_BRANCH_PREFIX` (`t3code`)
• `@t3tools/*` npm package names

Universal QA gate (automated)

• `bun fmt` clean (760 files)
• `bun lint` 0 errors (8 pre-existing warnings unrelated to this change)
• `bun typecheck` 0 errors across 8 packages
• `apps/desktop` Vitest: 81/81 passing

QA requiring manual / device verification

These are the rebrand-specific QA items from `docs/REBUILD_PLAN.md` that automated checks cannot cover. Please verify before merging:

Visual / brand audit

• macOS menu bar shows `ClayCode (Dev)` in dev / `ClayCode (Alpha)` in built
• Window title bar correct
• About dialog shows correct name + version
• Dock icon tooltip correct
• cmd+Tab app switcher label correct
• Notification banner shows correct sender name
• Spotlight search finds the app under the new name
• Finder "Get Info" on `.app` shows correct `CFBundleName`

Bundle integrity

• Bundle ID still `com.t3tools.t3code` per env (do NOT regress to single ID)
• Auto-updater feed URL still resolves correctly
• Existing installed app upgrades in place (no duplicate install)
• `codesign --verify --deep --strict --verbose=2 ClayCode (Alpha).app` passes

Cross-platform

• Linux `.AppImage` shows correct `Name=` in `.desktop` entry
• Windows `.exe` properties show correct `ProductName`

Regression

• All existing keyboard shortcuts (cmd+N, cmd+W, cmd+Q, etc.) still work
• Deep links resolve
• Settings persist across restart
• First-run wizard does not re-trigger for existing users

Build verification (recommended before merge)

```
bun run build:desktop
open apps/desktop/dist/mac-arm64/ClayCode\ \(Alpha\).app
```

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