fix(watchlist): normalize and dedupe persisted ids

[jony376]

Summary

Normalize and dedupe persisted watchlist IDs across all categories to prevent inflated badge counts and inconsistent watched-state behavior.

Changes

• Added ID normalization (trim + dedupe + empty removal) in watchlist storage read path.
• Applied normalization in V1->V2 migration path.
• Enforced normalization in add/toggle write paths to avoid future duplicate drift.
• Normalized starred PR keys in watched PR source filtering.

Related Issues

Closes #1300

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe):

Testing

• npm run build passes
• Manual browser smoke test

Not run locally in this pass.

Checklist

• Self-reviewed the diff
• Follows existing code patterns and naming
• No unrelated changes included
• Documentation updated if behavior changed

[anderdc]

The add/toggle paths already block duplicates via the existing includes() guard, and every watchlist id comes from structured API data — repo full names, hotkeys, serialized PR keys — that carries no whitespace. The duplicate/whitespace state this normalizes is only reachable by hand-editing localStorage, which is exactly the reproduction in #1300. No real-world path produces the drift. Closing.