feat: dedupe snapshots and filter review picker

[flexdinesh]

Repeated servediff review calls now reuse an unexpired context when source, checkout, branch, HEAD, and complete diff content match, preserving comments and reviewed marks.

• Hash all/staged/unstaged content before preview limits, including binary/untracked files, modes, renames, and conflicts; ignore filesystem timestamps.
• Expire stored diffs after seven days without a fresh submission. Matching fresh reviews reset TTL; exact retries do not. Prune all scopes and associated review state locally and remotely.
• Add Availability, Host, Changes, Branch, and Worktree picker filters; default Available, filter before grouping, and disable unavailable rows with keyboard skipping.
• Preserve existing duplicate histories and URLs until expiry. Unsupported full identities remain independent; older producers dedupe only empty snapshots.

Validation: mise run check (52 browser tests), mise run test:race, desktop/mobile picker inspection; regenerated API types and embedded assets committed.