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/Settings/SettingsGeneral.tsx

@@ -102,6 +102,13 @@ export function SettingsGeneral() {
               description="Periodically ping Yaak servers to check for relevant notifications."
             />
 
+            <ModelSettingRowBoolean
+              model={settings}
+              modelKey="hideFeedbackPrompts"
+              title="Never ask for feedback"
+              description="Hide the rare one-time prompts asking how new features are working."
+            />
+
             <SettingRowBoolean
               title="Send anonymous usage statistics"
               description="Yaak is local-first and does not collect analytics or usage data."