feat(share): promote Share out of Labs

[graydawnc]

What

The share editor (local export: image / PDF / Markdown / .spool) graduates from a Labs experiment to a standard always-on feature, following the Security Scan promotion (#420). This removes:

• The share flag: the VITE_FEATURE_SHARE env (dropped from the test:e2e script; release.yml never set it), the localStorage tri-state store (labsFlags.ts, spool.labs.share), and the resolveFeatureRuntime/useFeature machinery. featureFlags.ts now contains only the sharePublish build-time gate, which stays until the publish backend ops are ready — its removal is the launch switch.
• The entire Labs tab — with share promoted it had zero rows left. Tab definition, LabsTab component, settings.tab_labs + the labs.* i18n block across all 7 locales. Future experiments can reintroduce the scaffolding.
• Every shareEnabled gate: App.tsx view gates and ~10 conditional prop spreads, the flag-off bounce-navigation effect (unreachable once the flag is gone), ShortcutsTab/getShortcutGroups (share-editor shortcut group now static — folded into a SHORTCUT_GROUPS const), useDraftCountForSession.

The Labs tab hosted the app's only in-app feedback affordance (the Discord invite). It is relocated to Settings → General → About ("Give feedback", translated in all 7 locales) rather than dropped.

Behavior notes

• Users who had toggled Share off in Labs get it on after upgrading; stale spool.labs.share localStorage keys are inert (nothing reads them).
• Prod builds now ship the share editor on by default (previously opt-in via Labs). The remote publish/sign-in surface is unchanged — still gated by VITE_FEATURE_SHAREPUBLISH, which prod does not set.

Known coverage gap (documented, not new)

The shipped prod configuration is now share-on / publish-off, but every e2e build pins VITE_FEATURE_SHAREPUBLISH=1, so the publishEnabled=false arms (ShareMenu popover, SharesPage tabs, Settings rail) render in no automated test — covering them needs a second flag-less build in CI. Before this PR the gap was moot (share itself was off in prod); it is tracked in the share GA fast-follow pool ("flag-OFF render smoke") and should land before the publish flag flips.

Test plan

• Removed tests covered removed behavior: labsFlags.test.ts (tri-state store), the resolveFeatureRuntime truth table, and settings-labs.spec.ts (toggle persistence + flag-off bounce navigation). The Security-always-on assertion from that spec moved to settings.spec.ts.
• New e2e in settings.spec.ts: (1) sidebar-shares + sidebar-security visible with no env/opt-in and no settings-tab-labs in the rail; (2) the relocated feedback link renders in General → About with the Discord href.
• App unit suite green (447); full e2e suite green locally on macOS — all share-*.spec.ts suites now exercise the always-on path since the e2e build no longer sets VITE_FEATURE_SHARE.
• High-effort multi-agent review on the final diff: 3 findings — feedback-link loss (fixed here), the publish-off coverage gap (documented above), optional share-callback props left in consuming components (type-safety cleanup, deferred to a follow-up PR; zero runtime difference).

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