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.

docs/EVALS.md

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+# Project evaluations — LexAI
+
+> Small, repeatable checks. Prefer a deterministic command or test over a prose reminder. The standing checks below are the baseline gates for any change.
+
+## Standing gates (run on every change)
+
+### E-BASE-01 — Typecheck
+
+- **How to run:** `npm run typecheck`
+- **Pass condition:** `tsc --noEmit` exits 0.
+- **Cost:** fast.
+
+### E-BASE-02 — Unit tests
+
+- **How to run:** `npm test -- --run`
+- **Pass condition:** vitest exits 0.
+- **Note:** current unit tests exercise the `chrome.storage` mock, not the real handlers — passing does **not** prove provider routing or key decrypt. See TASKS #8.
+
+### E-BASE-03 — Production build
+
+- **How to run:** `npm run build`
+- **Pass condition:** builds to `.output/chrome-mv3/`; bundle roughly ~166 KB baseline.
+- **Cost:** fast (~3s).
+
+### E-BASE-04 — Manual real-page check (behavior changes)
+
+- **How to run:** `npm run build` → load unpacked `.output/chrome-mv3` → select text on a textarea and a contenteditable site → run an action → Replace.
+- **Pass condition:** toolbar shows, result modal returns, Replace edits both target types.
+- **Why manual:** selection/replace is DOM-timing-sensitive and has no automated coverage.
+
+## Active failure-derived checks
+
+_No failure-derived checks yet. Add one here when a verified regression gives a deterministic trigger — e.g. a guard that fails if the built `content.js` still contains `[LexAI` logs (T-03), or a test asserting the `onMessage` listener returns `true`._
+
+## Eval template
+
+```markdown
+### E-YYYY-MM-DD-NN — [short check name]
+- **Prevents:** [lesson ID and failure mode]
+- **How to run:** `[exact command or steps]`
+- **Pass condition:** [observable]
+- **Cost:** fast | moderate | expensive
+- **Last verified:** [date + result]
+```
+
+## Retired checks
+
+_Move obsolete checks here with the reason and the lesson they covered._