fix(announcement): key the forced read on an announcement ID, not the app version

[lshw54]

What

Updating the app re-forces the 30-second announcement read even when the announcement content has not changed. The seen state stores the acknowledged app version (announcementSeenVersion = "6.0.5"), so every release invalidates everyone's acknowledgement — the opposite of the intended behaviour: unchanged announcement → no re-read after update; changed announcement → forced read.

Fix

Identify the announcement by a content revision string instead of the app version:

• src/constants/announcement.ts (new) — ANNOUNCEMENT_ID ("YYYY-MM-description" format, currently 2026-07-dual-line-development-notice), ANNOUNCEMENT_FORCED_SECONDS, the two card URLs, and the isAnnouncementSeenValue() predicate. Publishing a new announcement = bump the ID (single edit; the file's doc comment documents the playbook) → resets every user's seen state. App updates alone never re-force an unchanged notice.
• AnnouncementModal.vue — compares/persists ANNOUNCEMENT_ID instead of the app version (commands.version() call dropped). The storage key stays announcementSeenVersion so legacy acknowledgements remain readable.
• Migration — values written by pre-ID builds are app versions ("6.0.5"), and everyone who acknowledged one has read the current (issue 📢 【公告】關於 Beanfun 與 MapleLink 雙線並行開發與未來規劃說明 #323) notice — the only announcement that has existed. Version-shaped values therefore count as seen while the shipped ID is still the inaugural one; the clause self-deactivates automatically on the first real ID bump. Existing users are not innocently re-forced by this mechanism change, and future announcements still force everyone exactly once.

Tests

• Modal specs moved to the ID mechanism, plus two new cases: legacy app-version value suppresses the forced read; a different (bumped) announcement ID forces it.
• New predicate spec covering ID match, legacy values, near-miss version strings, and empty values.
• 646 frontend tests, typecheck, eslint, prettier all pass. No Rust changes.