chore(release): 0.13.0

[benvinegar]

Summary

Prepare Sideshow 0.13.0:

• promote the collapsible desktop sidebar as the release's minor feature
• include compact session hydration and live post updates
• include coalesced live session refreshes, narrow post counts, and SQLite hot-path indexes
• include the multi-viewer iframe sizing fix and updated Pi extension contract
• include audited dependency updates with no known vulnerabilities
• consume all pending Changesets and update CHANGELOG.md, package.json, and package-lock.json

Validation

• npm test
• npm run coverage
• npm run test:worker
• npm run typecheck
• npm run lint
• npm run format:check
• npm run security:audit
• Chromium Playwright: 88 passed
• node scripts/check-release-version.ts v0.13.0
• npm pack --dry-run
• independent release-note and version review: no blockers

Local WebKit could not launch because the host lacks Playwright's required WebKit libraries; the release PR's CI runs Chromium and WebKit with managed dependencies.

No release tag has been created. After this PR merges, tag the resulting main commit as v0.13.0 to publish.

This PR description was generated by Pi using GPT-5.6 Sol