Add InvariantError class and assertion utilities for runtime validation:
- invariant(condition, message) - throws if condition is falsy, acts as type guard
- assertNever(value) - for exhaustive switch statement checking
- Support for lazy message evaluation to avoid expensive string operations
These helpers enable defensive programming with clear error messages when
invariants are violated.
Closes bd-1w5
- Log swallowed errors in file watcher and window operations (lib.rs, watcher.rs)
- Propagate recovery errors from bridge::recover_pending to SyncResult.errors
so the frontend can display them instead of silently dropping failures
- Fix useTauriEvent/useTauriEvents race condition where cleanup fires before
async listen() resolves, leaking the listener (cancelled flag pattern)
- Guard computeStaleness against invalid date strings (NaN -> 'normal'
instead of incorrectly returning 'urgent')
- Strengthen isMcError type guard to check field types, not just presence
- Log warning when data directory resolution falls back to '.' (state.rs, bridge.rs)
- Add test for computeStaleness with invalid date inputs
