Drop the legacy local practice Y.Doc loader and its Dexie tables

[InfinityBowman]

The local practice plane keeps two generations of legacy data machinery whose rollback windows have closed. With the schema-migration replay in place (the PR stacked on #800), local rows follow the same mechanism as online rows, and this is the remaining local-only surface.

What to remove

• packages/web/src/project/localProject.ts: everything except LOCAL_PROJECT_ID and LOCAL_REVIEWER_ID. The localChecklists flat-row import (migrateLocalChecklistsToYDoc, the buildStudyForLocalRow family, the text seeding), and loadLegacyLocalRows. Move the two constants somewhere without the yjs and y-dexie imports.
• rowsFromLocalDoc in localCollections.ts, and the yjs import there.
• packages/web/src/project/localProject.test.ts (tests the round-trip of the flat-row import).
• packages/web/src/dev/legacy.ts and its window.__devLegacy seam in dev/expose.ts.
• packages/web/e2e/flat-key-migration.spec.ts.
• packages/web/src/primitives/useProject/flatKeyMigration.ts and the checklist handlers under primitives/useProject/handlers/ if nothing else imports them (the commit that moved the write path onto the outbox kept them only for the legacy conversion).
• The ConnectionPool.initializeLocalRows fallback that calls the loader when no localProjects row exists. A device with no row starts empty.

Dexie

• Add a schema version that drops the projects ydoc table and localChecklists (this.version(5).stores({ projects: null, localChecklists: null })).
• Remove the yDexie addon and the y-dexie dependency once no table declares a Y.Doc column. Check whether Dexie needs the addon to parse the historical version 1 declaration on existing databases before removing it.
• Update db.on('ready') and clearAllData, which reference the projects table and the local Y.Doc row, and the glossary entries for projects and localChecklists.
• localChecklistPdfs stays: it is live storage for practice PDFs.

Risk

Any device that has not opened the app since the row-plane cutover (the localProjects table shipped in Dexie version 3) would lose local practice data it never converted. Decide on a cutoff and say so in the release notes.

Refs #791.

[InfinityBowman]

Carried over from #533, which this supersedes. One item there is not covered above:

Backups

• Delete the pre-freeze export from R2 (corates-pdfs-prod/cutover-backup/2026-08-08/) and the local copies in ~/Documents/corates-cutover-backups/

Same rationale as the rest of this cleanup: the rollback window has closed. It is independent of the code removal and can be done separately.