feat: Implement Prompt Builder functionality in Popup and Options

- Added a new "Prompt Builder" tab in the Popup for generating AI prompts with customizable parameters.
- Introduced new state variables for managing prompt styles, personas, formats, and models.
- Enhanced the Options page to fetch and display models based on the provided API key.
- Updated the actions and types to include the new 'prompt' action and its associated parameters.
- Implemented migration logic for legacy plaintext API keys to encrypted storage.
- Updated the getSystemPrompt function to incorporate prompt parameters for better instruction generation.
- Added tests for the new functionality, including context menu entries and prompt generation logic.

src/lib/actions.ts

@@ -4,7 +4,7 @@
 
 // 'fix' is the user-facing id emitted by the context menu and popup; the
 // background normalizes it to the 'grammar' prompt.
-export const ACTIONS = ['fix', 'rephrase', 'shorten', 'expand', 'explain'] as const;
+export const ACTIONS = ['fix', 'rephrase', 'shorten', 'expand', 'explain', 'prompt'] as const;
 export type ActionId = (typeof ACTIONS)[number];
 
 export const ACTION_LABELS: Record<ActionId, string> = {
@@ -13,6 +13,7 @@ export const ACTION_LABELS: Record<ActionId, string> = {
   shorten: 'Shorten',
   expand: 'Expand',
   explain: 'Explain',
+  prompt: 'Make Prompt',
 };
 
 // Full list, including the 'Default' pseudo-style (= no style modifier).
@@ -22,6 +23,37 @@ export type WritingStyle = (typeof WRITING_STYLES)[number];
 // Context menus omit 'Default' — the parent action item already covers it.
 export const CONTEXT_MENU_STYLES = WRITING_STYLES.filter((s) => s !== 'Default');
 
+// ─── Prompt Builder parameters (the 'prompt' action) ─────────────────────────
+// 'Auto' always means "let the prompt engineer decide from the input".
+
+export const PROMPT_STYLES = ['Auto', 'Instructional', 'Role-play', 'Step-by-step', 'Few-shot', 'Structured'] as const;
+export type PromptStyle = (typeof PROMPT_STYLES)[number];
+
+// 'Custom…' switches the popup to a free-text persona field.
+export const PROMPT_PERSONAS = [
+  'Auto',
+  'None',
+  'Expert Developer',
+  'Copywriter',
+  'Teacher',
+  'Data Analyst',
+  'Business Consultant',
+  'Researcher',
+  'Custom…',
+] as const;
+export type PromptPersona = (typeof PROMPT_PERSONAS)[number];
+
+export const PROMPT_FORMATS = ['Auto', 'Plain text', 'Markdown', 'Bulleted list', 'Numbered steps', 'JSON', 'Table'] as const;
+export type PromptFormat = (typeof PROMPT_FORMATS)[number];
+
+// 'Custom…' means "use the free-text persona"; an empty custom text falls
+// back to Auto. Shared by the popup and the background (which resolves saved
+// Prompt Builder settings for toolbar/context-menu prompt requests).
+export function resolvePromptPersona(persona?: string, customPersona?: string): string | undefined {
+  if (persona === 'Custom…') return customPersona?.trim() || 'Auto';
+  return persona;
+}
+
 // Minimum selection length (chars, after trim) before the floating toolbar
 // appears. Kept deliberately above 1-2 chars so accidental double-click
 // selections don't trigger the UI.
@@ -49,13 +81,17 @@ export const CONTEXT_MENU_ENTRIES: ContextMenuEntry[] = ACTIONS.flatMap((action)
     style: 'Default' as WritingStyle,
     title: `⚡ LexAI: ${ACTION_LABELS[action]}`,
   },
-  ...CONTEXT_MENU_STYLES.map((style) => ({
-    id: `lexai-${action}-${style.toLowerCase()}`,
-    action,
-    style,
-    parentId: `lexai-${action}`,
-    title: style as string,
-  })),
+  // 'prompt' opens the Prompt Builder dialog in the page, which has its own
+  // parameters — writing-style children don't apply to it.
+  ...(action === 'prompt'
+    ? []
+    : CONTEXT_MENU_STYLES.map((style) => ({
+        id: `lexai-${action}-${style.toLowerCase()}`,
+        action,
+        style,
+        parentId: `lexai-${action}`,
+        title: style as string,
+      }))),
 ]);
 
 export function findContextMenuEntry(menuItemId: string): ContextMenuEntry | undefined {