Liquid Glass theme with in-app appearance switcher (#1337)

* Centralize UI theme colors into semantic ThemePalette tokens

Introduce AppTheme/ThemePalette (Utils/Theme.swift) with an environment
key and @ThemedPalette property wrapper, persisted via @AppStorage.
Views now resolve background/primary/secondary/accentBlue/alertRed/
divider tokens from the environment instead of computing colors inline,
removing the backgroundColor/textColor/secondaryTextColor prop-drilling
through the header, composer, and people-sheet hierarchy.

Matrix theme output is pixel-identical; this is groundwork for a
user-selectable theme switcher.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Add liquid glass theme with in-app appearance switcher

Add a .liquidGlass AppTheme alongside the matrix terminal theme,
selectable from a one-line APPEARANCE row in the app info sheet
(persisted via @AppStorage, applies live).

Liquid glass renders system fonts and colors over a subtle gradient
backdrop, with the header and composer floating as Liquid Glass panels
(real .glassEffect() on iOS/macOS 26, compiler-gated with an
ultraThinMaterial fallback for older SDKs). The message list scrolls
underneath the chrome via safe-area insets, and all sheets share the
same backdrop and surface language. The matrix theme is unchanged.

Details:
- Theme is threaded through ChatMessageFormatter so message
  AttributedStrings switch font design per theme; the per-message
  format cache gains a variant key so themes never serve each other's
  cached strings
- New palette tokens: accent (interactive tint) and locationAccent
  (geohash green), replacing scattered hardcoded greens/blues in the
  voice note, waveform, verification, and people-sheet views
- Header controls get full-height tap targets and the people count
  becomes a real Button (previously a tap gesture on the cluster)
- CommandSuggestionsView renders nothing when empty instead of a
  zero-height view that pushed the composer input off-center
- New appearance strings localized for all 29 catalog languages

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: jack <jackjackbits@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

bitchat/Views/MessageListView.swift

@@ -21,6 +21,7 @@ struct MessageListView: View {
     @EnvironmentObject private var locationChannelsModel: LocationChannelsModel
 
     @Environment(\.colorScheme) private var colorScheme
+    @Environment(\.appTheme) private var theme
 
     let privatePeer: PeerID?
     @Binding var isAtBottom: Bool
@@ -222,7 +223,7 @@ private extension MessageListView {
 
     @ViewBuilder
     func systemMessageRow(_ message: BitchatMessage) -> some View {
-        Text(conversationUIModel.formatMessage(message, colorScheme: colorScheme))
+        Text(conversationUIModel.formatMessage(message, colorScheme: colorScheme, theme: theme))
             .fixedSize(horizontal: false, vertical: true)
             .frame(maxWidth: .infinity, alignment: .leading)
     }