feat(update): integrate shared studio update dialog behind flag

[ElbertePlinio]

Summary

• Mounts @pickforge/tauri-updater's shared controller + <pickforge-update-dialog> element on PickGauge's main window when the studioUpdateDialog flag is on. Default off (@pickforge/flags, src/lib/flags.ts) — no PickGauge-local flag registry existed before this PR, so a thin per-app wrapper was added matching the pattern already used in pickforge/src/stores/flags.ts (own definitions per app repo, not a monorepo-shared registry).
• While the flag is off, the legacy window.confirm updater in src/lib/updater.ts is untouched (removal is a later PR per the issue's rollout plan).
• src/lib/updateDialog.ts enforces the hard rules from the issue:
  • Capsule exclusion: createMainWindowEligibility checks the real Tauri window label (win.label !== "main") — the floating capsule (Float.svelte, label "float") never imports this module at all, and even if it did, the label check independently blocks it.
  • Hidden-window exclusion: a main window that isn't visible yet defers via onFocusChanged, resolving eligible only on first focus — this mirrors the pre-existing checkForUpdatesWhenVisible deferral in updater.ts so behavior doesn't regress.
  • Single check per process: one memoized controller promise plus the package's createProcessCheckGate().
  • Packaged builds only: no-ops in dev and outside Tauri (browser preview), matching the legacy flow.
  • Silent, non-blocking startup failures: wiring errors are caught and logged; nothing blocks app startup.
• main.ts skips the legacy checkForUpdatesWhenVisible call on the main window only when the flag is on, so the two flows can never double-prompt.
• Added a dev-only ?updateDialogFixture=available|downloading route (src/lib/updateDialogFixture.ts) with injected deterministic state for manual visual inspection at PickGauge's normal (1000×700) and minimum (820×580) sizes. PickGauge has no VRT/screenshot-baseline harness (confirmed — no such workflow or fixture infra exists in this repo), so per the issue's fallback this fixture is kept/polished instead of building one; import.meta.env.DEV elimination confirmed by build (fixture code appears only in the sourcemap, not the executable bundle).
• docs/releases/UNRELEASED.md updated (internal/release change entry + validation).

Fixed from the inherited work

• @pickforge/flags and @pickforge/tauri-updater were pinned exact (0.4.0, 0.12.0) in package.json/bun.lock. Every other @pickforge/* dependency across PickForge/PickScribe/PickGauge uses a caret range except @pickforge/brand (deliberately exact-pinned for asset consistency). Changed both to ^0.4.0 / ^0.12.0 to match the workspace convention and re-synced the lockfile.

Validation

• bun run lint — clean
• bun run check (svelte-check) — 0 errors, 0 warnings
• bun run test — 85 vitest tests (incl. new flags.test.ts, updateDialog.test.ts) + 18 Node tests, all passing
• bun run test:coverage — ratchet holds (existing thresholds: 16/23/26/17)
• bun run build — succeeds; confirmed the dev-only fixture module is tree-shaken from the production bundle
• No Rust files touched; cargo check/cargo test not run

Owner-gated packaged-update smoke (old build → staged newer build) and floating/hidden-window smoke on a real packaged app are deferred, per the issue's PR plan, to the flag-enable PR later in the rollout.

Refs pickforge/pickforge-platform#36

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