feat: add usage panel with session, weekly, and monthly cost bars

Scan ~/.claude/projects JSONL files to compute API cost usage.
Shows 4 bars matching Claude's usage screen: current session (5h window),
weekly all models, weekly sonnet-only, and monthly total.
Configurable plan limits in PLAN_LIMITS. Refreshes every 30s.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -35,3 +35,6 @@ report.[0-9]_.[0-9]_.[0-9]_.[0-9]_.json
 
 # Asset generator (not needed for users)
 assets/gen-screenshots.py
+
+# Reference repos
+_reference/