feat: expose flicker-free mode as a /config toggle (closes #260) (#265)

Add flickerFreeMode to GlobalConfig so external users can enable
fullscreen alt-screen mode via /config instead of having to set
the CLAUDE_CODE_NO_FLICKER=1 env var manually.

Priority order in isFullscreenEnvEnabled():
  CLAUDE_CODE_NO_FLICKER=0  → always off (env wins)
  CLAUDE_CODE_NO_FLICKER=1  → always on (env wins)
  tmux -CC detected         → off (terminal safety guard)
  config flickerFreeMode    → user preference (new)
  USER_TYPE=ant             → internal default

The env var still takes full precedence so existing scripts and
automation are unaffected. The new setting only activates when
flickerFreeMode is explicitly set in config.

src/utils/config.ts

@@ -464,6 +464,11 @@ export type GlobalConfig = {
   // Fullscreen in-app text selection behavior
   copyOnSelect?: boolean // Auto-copy to clipboard on mouse-up (undefined → true; lets cmd+c "work" via no-op)
 
+  // Flicker-free fullscreen mode (equivalent to CLAUDE_CODE_NO_FLICKER=1 env var).
+  // When true, enables alt-screen + virtualized scroll for all users.
+  // Env var still takes precedence: =0 always off, =1 always on.
+  flickerFreeMode?: boolean
+
   // GitHub repo path mapping for teleport directory switching
   // Key: "owner/repo" (lowercase), Value: array of absolute paths where repo is cloned
   githubRepoPaths?: Record<string, string[]>
@@ -659,6 +664,7 @@ export const GLOBAL_CONFIG_KEYS = [
   'lspRecommendationIgnoredCount',
   'copyFullResponse',
   'copyOnSelect',
+  'flickerFreeMode',
   'permissionExplainerEnabled',
   'prStatusFooterEnabled',
   'remoteControlAtStartup',