Add in-app micro-feedback prompts for new features

Show a one-time toast asking how a feature is working after its third
successful use, with an optional comment sent anonymously to the Yaak
API (feature key, text, app version, and OS only — nothing identifying,
and nothing is sent unless the user clicks Send).

- New cmd_send_feedback Tauri command posts fire-and-forget via the
  shared API client (localhost in dev)
- Feature keys registry (cookie-editor, response-history, sse-summary,
  git-sync) with per-feature use counting in the key-value store
- "Never ask for feedback" setting to disable prompts entirely
- Toast gains dynamicHeight and hideDismiss props for richer content
- Fix missing vertical padding on xs/2xs multiline inputs
- Fix unused-import and dead-code warnings in yaak-system-appearance
  on non-Linux builds

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

apps/yaak-client/lib/tauri.ts

@@ -48,6 +48,7 @@ type TauriCmd =
   | "cmd_save_response"
   | "cmd_secure_template"
   | "cmd_send_ephemeral_request"
+  | "cmd_send_feedback"
   | "cmd_send_http_request"
   | "cmd_template_function_summaries"
   | "cmd_template_function_config"