fix: recover from interrupted session restores

[PathGao]

Fixes #259.

Summary

• Mark automatic session restoration as in progress before restored tabs are read and rendered.
• Clear the marker after normal completion or a handled restore failure.
• On the next startup after an interruption, discard only the saved-tab snapshot and start normally; user documents are untouched.

Validation

• node --test --import tsx scripts/interruptedSessionRestore.test.ts
• npm run check

[PathGao]

@alecdotdev This branch has been rebased onto the latest #258 branch and force-pushed; the merge conflict is resolved. Validation passed: node --test --import tsx scripts/interruptedSessionRestore.test.ts scripts/windowStateRestore.test.ts and npm run check. Please merge in this order: #254 → #258 → #260 → #266.