refactor(sync): extract BaseItemHandler<T> + migrate 13 handlers (Phase 4 U8)

[h4yfans]

Summary

Phase 4 U8 of tech-debt-remediation: extract a shared BaseItemHandler<T> abstract class and migrate all 13 sync item handlers to inherit from it. Eliminates ~40 lines of boilerplate per handler (default fetchLocal/seedUnclocked stubs + unused SyncItemHandler type imports + scattered resolveClockConflict re-imports) while preserving every handler's behavior.

• New: apps/desktop/src/main/sync/item-handlers/base-handler.ts — abstract class with type/schema/applyUpsert abstract, sensible defaults for applyDelete/fetchLocal/seedUnclocked, and a protected resolveClock() helper.
• Migrated (12 extend BaseItemHandler): calendar-binding, calendar-event, calendar-external-event, calendar-source, filter, folder-config, inbox, journal, note, project, tag-definition, task.
• Standalone (1): settings-handler implements SyncItemHandler directly with an inline comment — it overrides every concrete method because settings live in config.json + prefs cache, not a sync DB table. Inheritance would add indirection for zero gain.
• Untouched: types.ts, index.ts, field-merge.ts. SyncItemHandler<T> interface shape unchanged. getHandler(type) / getAllHandlers() signatures unchanged.

LOC delta

14 files changed, +219 / -147 (net +72, of which ~40 is the new base-handler.ts itself).

Test plan

• pnpm test (desktop via turbo) — 5872 passed, 1 pre-existing skip, 0 failures
• pnpm --dir apps/desktop exec tsc --noEmit -p tsconfig.node.json — clean
• pnpm --dir apps/desktop exec tsc --noEmit -p tsconfig.web.json — clean
• pnpm ipc:check — RPC bindings + invoke map up to date
• Manual smoke (post-merge): launch desktop app, create/edit/delete items across types, verify sync push+pull round-trip

Notes

• All existing handler-level tests (settings-handler.test.ts, note-handler.test.ts, task-handler.test.ts, project-handler.test.ts, folder-config-handler.test.ts, calendar-event-handler.test.ts, note-handler-binary.test.ts) pass without modification — they test the exported singleton's methods, and the singleton's shape is preserved.
• Each handler now calls this.resolveClock(...) instead of the free resolveClockConflict import. Behavior is identical — resolveClock on the base just delegates.
• No changes to runtime semantics, transaction boundaries, logging, or emitted events.