fix: address CodeRabbit review — error handling, displayName, progress math

[devin-ai-integration]

Summary

Addresses the 3 CodeRabbit review comments from PR #68:

1. handleSignOut error handling — wrapped signOut(auth) in try-catch so network/Firebase failures don't crash the component. navigate("/") only runs on success.

2. displayName initials edge case — added .filter((n) => n.length > 0) before .map((n) => n[0]) so consecutive spaces in displayName (e.g. "John Doe") don't produce undefined in the initials string.

3. overallProgress hardening — null-safe project.stages ?? {} access, Number.isFinite validation on each completionPercent, and divide-by-zero guard when stageValues.length === 0.

Link to Devin session: https://app.devin.ai/sessions/3cc968b3f6af4fc09059e162d7190a95
Requested by: @hrx01-dev

[devin-ai-integration]

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