fix(auth): Google Sign-In audience mismatch + remove per-user accent color

Root cause of "Google authentication failed": appsettings.Development.json
had Auth:GoogleClientId set to the literal placeholder
"CHANGE_ME_GOOGLE_CLIENT_ID" while the frontend's .env.development had a
real (already-public, already-committed) client ID -- every Google ID
token's audience check failed against the backend's placeholder. Fixed
by setting the same real client ID on both sides (a client ID is a
public identifier, not a secret, safe to commit -- unlike a client
secret). Also enabled Auth:AllowRegistration in dev so the existing
Google-first self-serve-signup path (auto-create on unmatched verified
email, auto-link on matching verified email -- built during Wave 7) is
actually exercisable locally.

Wired the previously-missing Auth__MicrosoftClientId /
NEXT_PUBLIC_MICROSOFT_CLIENT_ID into docker-compose.yml/.env.example
(distinct from the existing MICROSOFT_CLIENT_ID used for Outlook mail
linking) -- Microsoft sign-in was never deployable, a leftover gap from
when it was built. Fixed a stale env-var name in the Microsoft setup
hint copy (still said REACT_APP_*, predates the Next.js migration).

Removed the per-user accent color picker entirely: it was purely
client-side (localStorage + theme.ts), never touched the backend/DB.
theme.ts now hardcodes a single ACCENT constant; themePrefs.ts drops
get/set/clearAccentColor; App.tsx and SettingsView.tsx drop the
accentColor prop threading. Dead accent-related i18n keys removed from
both locales.

Consolidated Settings' "Account" tab (duplicated GoogleAuthCard, which
already lives on the Profile page) into Profile: moved AuthStatusCard
and EmailProviderConnections there alongside the existing Google/
Microsoft auth cards, so identity/account-linking lives in one place.
Settings drops from 5 tabs to 4 and its General tab uses a consistent
SectionCard layout instead of ad-hoc per-card styling.

Verified: dotnet build/test (177/177) and npm build/test (57/57) both
green; confirmed live against a running dev server that /auth/config
now reports googleEnabled with the corrected client ID, Settings has
no accent controls, and Profile shows the consolidated auth section.

.env.example

@@ -5,6 +5,9 @@ AUTH_JWT_KEY=CHANGE_ME_LONG_RANDOM_SECRET
 AUTH_ADMIN_EMAIL=admin@example.com
 AUTH_ADMIN_PASSWORD=CHANGE_ME_STRONG_PASSWORD
 AUTH_GOOGLE_CLIENT_ID=CHANGE_ME_GOOGLE_CLIENT_ID
+# Optional: enables the "Continue with Microsoft" sign-in tab (separate from the
+# MICROSOFT_CLIENT_ID below, which is for Outlook mail linking, not sign-in).
+AUTH_MICROSOFT_CLIENT_ID=
 GOOGLE_GMAIL_CLIENT_SECRET=CHANGE_ME_GOOGLE_OAUTH_CLIENT_SECRET
 # Optional. If omitted, the backend uses https://<your-domain>/api/gmail/oauth/callback
 GOOGLE_GMAIL_REDIRECT_URI=