Add cancel, history, revert, and quality-of-life features

UX

- Add elapsed-time counter and Cancel button to in-flight generations,
  wired through AbortController so partial responses are discarded
  cleanly. The cancel action is available from both the input panel
  and the results header.
- Add a Recent generations panel below the input. The last 6 successful
  generations are saved to localStorage; one click reloads both the
  input fields and the generated assets.
- Add a Revert button to every editable card. It appears the moment
  the current value diverges from the last generated value and restores
  the field with a single click.
- Add an Empty state to the right panel with a friendly hint pointing
  at the Generate button and the Settings page.
- Add a 'Try an example' button that fills the input with a random
  starter idea (idea + mood + vocals).
- Add Cmd/Ctrl+Enter as a keyboard shortcut to generate.
- Add a Footer with project info and a privacy reminder.
- Add a 'Clear saved key' button to the Settings page so the user can
  remove the API key without overwriting it.

Bug fix

- Settings > Test Connection used to save the in-progress form values
  to localStorage before testing. It now uses the in-memory candidate
  config, so failed tests don't pollute the saved config.

Code quality

- Extract the duplicated useAutoHeight hook to src/lib/useAutoHeight.ts.
- Extract a useElapsed hook for the loading timer.
- Move InputValues into src/lib/types.ts (was duplicated in
  InputPanel.tsx) and add SECTION_LABELS, replacing the humanizeSection
  switch in HomePage.
- Centralize the filename sanitization: HomePage now calls
  sanitizeFilename from zip.ts instead of duplicating the regex.
- Add previewConfig / testConnectionWithConfig helpers to llm.ts to
  support in-memory connection tests.

src/lib/types.ts

@@ -1,27 +1,27 @@
 // Domain types shared across the app. These mirror the JSON shape produced by
-// the `generate-song` edge function.
+// the LLM module.
 
 export type Language =
-  | 'English'
-  | 'Deutsch'
-  | 'Español'
-  | 'Français'
-  | 'Italiano'
-  | 'Português'
-  | 'Polski'
-  | 'Other';
+  | "English"
+  | "Deutsch"
+  | "Español"
+  | "Français"
+  | "Italiano"
+  | "Português"
+  | "Polski"
+  | "Other";
 
-export type Vocals = 'vocals' | 'instrumental';
+export type Vocals = "vocals" | "instrumental";
 
 export type GenerationSection =
-  | 'all'
-  | 'lyrics'
-  | 'style'
-  | 'titles'
-  | 'video_prompts'
-  | 'youtube_description';
+  | "all"
+  | "lyrics"
+  | "style"
+  | "titles"
+  | "video_prompts"
+  | "youtube_description";
 
-export type VideoPromptType = 'Abstract' | 'Cinematic' | 'Hybrid';
+export type VideoPromptType = "Abstract" | "Cinematic" | "Hybrid";
 
 export interface VideoPrompt {
   type: VideoPromptType;
@@ -40,7 +40,7 @@ export interface SongAssets {
 
 export interface GenerateRequest {
   input: string;
-  language: string;          // 'English' or free-text from "Other"
+  language: string; // 'English' or free-text from "Other"
   mood?: string;
   vocals: Vocals;
   section: GenerationSection;
@@ -53,16 +53,29 @@ export interface ConfigDisplay {
   api_key_set: boolean;
 }
 
-export interface SaveConfigPayload {
-  api_endpoint: string;
-  api_key: string; // empty string => keep existing
-  model_name: string;
-}
-
 export interface TestConnectionResult {
   success: boolean;
   message: string;
 }
 
+export interface InputValues {
+  idea: string;
+  language: Language | string;
+  customLanguage: string;
+  mood: string;
+  vocals: Vocals;
+}
+
 export const NEGATIVE_VIDEO_PROMPT =
-  'text, watermark, logo, flash, abrupt cuts, camera shake, faces, hands, distortion, blur';
+  "text, watermark, logo, flash, abrupt cuts, camera shake, faces, hands, distortion, blur";
+
+// Human-readable labels for each regenerable section — used by toasts and the
+// history panel. Keep in sync with `GenerationSection`.
+export const SECTION_LABELS: Record<GenerationSection, string> = {
+  all: "everything",
+  titles: "titles",
+  lyrics: "lyrics",
+  style: "style",
+  video_prompts: "video prompts",
+  youtube_description: "description",
+};