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/components/CookieDialog.tsx

@@ -13,6 +13,7 @@ import {
   useState,
 } from "react";
 import { showDialog } from "../lib/dialog";
+import { trackFeatureUsage } from "../lib/featureFeedback";
 import { jotaiStore } from "../lib/jotai";
 import { cookieDomain } from "../lib/model_util";
 import {
@@ -131,6 +132,7 @@ export const CookieDialog = ({ cookieJarId }: Props) => {
     });
 
     void patchModel(cookieJar, { cookies: [...nextCookies, nextCookie] });
+    trackFeatureUsage("cookie-editor");
     setSelectedCookieKey(nextCookieKey);
     setEditingCookieKey(null);
     setDraftCookie(null);