feat(theme): ship exactly 2 themes (SRC default + VNClagoon); rebrand user-facing VNCmail+

Product decision 2026-08-05: exactly SRC (default) and VNClagoon ship as
selectable themes. Everything else (Qui, Nord, Catppuccin, Solarized,
Roundcube Elastic, Aurora Glass) is hidden via ThemePolicy.disabledBuiltinThemes
rather than deleted - cheap to re-enable later, zero code lost. Also removed
the hardcoded "Default/Bulwark" theme card from the Settings > Themes grid
(product wants exactly 2 theme choices, not 3).

Separately, found and fixed real "still says Bulwark" branding gaps:
- All 24 locale files: "Bulwark"/"Bulwark Mail"/"Bulwark Webmail" ->
  "VNCmail+" in every user-facing string (verified via the translations
  test, which only checks structural key parity across locales, not
  content - a straight string swap is safe against it. 48/48 still pass).
- PWA manifest fallback app name, package.json description/author.
- Demo mode fixtures: the "Welcome to Bulwark Mail!" email and identity
  signature a first-time demo user actually sees.
- The demo empty-state's logo was hardcoded to a literal Bulwark SVG file
  regardless of active theme - a real bug, not just stale text, since
  lib/theme-logo.ts's resolveThemeLogo() already exists and is already used
  correctly by the login page and nav rail for exactly this (theme-aware
  SRC mark / VNClagoon wordmark). Wired the same helper in here instead of
  a hardcoded path.

NOT touched: internal code comments referencing "Bulwark" as historical/
attribution context (e.g. design-rationale comments explaining why a value
differs from Bulwark's original) - those are harmless and a full-repo sweep
of every comment wasn't the ask.

Verified: typecheck clean, lint clean, translations test 48/48 passing.

components/settings/themes-settings.tsx

@@ -72,21 +72,11 @@ export function ThemesSettings() {
 
       {/* Theme Grid */}
       <div className="grid grid-cols-2 sm:grid-cols-3 gap-3">
-        {/* Default theme card */}
-        <ThemeCard
-          name="Default"
-          author="Bulwark"
-          isDefaultTheme
-          variants={['light', 'dark']}
-          isDark={isDark}
-          isActive={activeThemeId === null}
-          isBuiltIn
-          isDefault={!themePolicy.defaultThemeId}
-          disabled={Boolean(forcedThemeId)}
-          onActivate={() => handleActivate(null)}
-        />
-
-        {/* Installed themes */}
+        {/* No "Default/Bulwark" card: product decision 2026-08-05 ships exactly
+            two themes (SRC default, VNClagoon) - see DEFAULT_THEME_POLICY in
+            lib/admin/types.ts. The underlying activateTheme(null) capability
+            stays reachable programmatically (e.g. an admin clearing
+            defaultThemeId), just not offered as a selectable card here. */}
         {visibleThemes.map(theme => {
           const isForceEnabled = theme.id === forcedThemeId || theme.forceEnabled || isThemeForceEnabled(theme.id);
           return (