refactor(auth): remove dead Gmail OAuth code path

Gmail authenticates via app password + preconfigured IMAP/SMTP (decision
in #9), never OAuth, so the Gmail-OAuth implementation was unreachable
dead code. Remove it while keeping Outlook's AppAuth OAuth path intact.

- Delete auth/GmailAuthManager.kt (shared OAuthResult/FreshToken kept).
- Drop AuthType.OAUTH_GMAIL and its exhaustive `when` branch, the
  gmailAuthManager injection, and the SCOPE_GMAIL constant in
  MailConnectionFactory.
- Remove GMAIL_OAUTH_* BuildConfig fields, gmailOAuthClientId, and the
  gmailRedirectScheme val from app/build.gradle.kts.
- Repoint the AppAuth manifestPlaceholders["appAuthRedirectScheme"] to
  the Outlook scheme (org.libremail.outlook). AppAuth's bundled manifest
  now registers that scheme on RedirectUriReceiverActivity, so the app
  manifest's now-redundant Outlook intent-filter is removed; the
  AppCompat theme override (crash fix) is preserved. Verified the merged
  manifest.
- Drop GMAIL_OAUTH_CLIENT_ID from secrets.properties.example.

authType persists as the enum name in a TEXT column, so removing a
constant needs no Room schema change or migration.

Closes #39

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

secrets.properties.example

@@ -1,11 +1,6 @@
-# Copy this file to `secrets.properties` (which is git-ignored) and fill in the value.
+# Copy this file to `secrets.properties` (which is git-ignored) and fill in the values you need.
+# Every value below is optional — the build works with the defaults when this file is absent.
 #
-# Gmail OAuth 2.0 *Android* client ID created in Google Cloud Console.
-# See the README ("Gmail account setup") for the exact steps. Used by the app for
-# the Authorization Code + PKCE login flow; no client secret is required for an
-# installed Android app.
-GMAIL_OAUTH_CLIENT_ID=
-
 # Optional: Microsoft (Outlook) OAuth public client id. A working default ships with the build;
 # set this only to use your own Azure app registration.
 #OUTLOOK_OAUTH_CLIENT_ID=