fix: refactor combat tracker buttons for better end combat confirmation flow

fix: fix CSP and COOP headers to allow Firebase signInWithPopup

fix: switch to signInWithPopup — eliminates getRedirectResult cold-load errors

fix: skip getRedirectResult entirely on cold load — only call if sign-in was initiated

fix: only show redirect sign-in errors if user initiated sign-in this session

fix: switch back to signInWithRedirect — popup blocked on Cloud Run domain

fix: improve auth error logging with error code and better hints

fix: revert to signInWithPopup now that domain is authorized

fix: improve auth error banner with actionable domain hint

fix: surface Google sign-in redirect errors in login UI

fix: switch Google sign-in from popup to redirect for production compatibility

feat: update branding text on LoginScreen for clarity and consistency

docs: comprehensive audit and update — sync all docs to current codebase state (e0300c3)

feat: add aria-label to file input for improved accessibility

feat: DM combat tracker, notes CRUD, character JSON import/export