Implement the following plan:

# Plan: Anonymous User Writing Pipeline Persistence

## Context
Anonymous users (not logged in) can write ankys today — the full pipeline works (cookie-based user ID, no auth guards). But if the browser closes or cookies are cleared, the user loses:
1. Their `anky_user_id` cookie → can't find their writings
2. The `currentAnkyId` JS variable → can't see their anky being generated
3. Any in-progress writing between 30-second checkpoint intervals

We need localStorage persistence so anonymous users never lose their work, and can recover it even after closing the browser.

## Changes

### 1. `templates/home.html` — Client-side localStorage persistence

**a) Save user token to localStorage on page load:**
- Read `anky_user_id` from cookie (it's not httpOnly, so JS can access it)
- Store it in `localStorage.setItem('anky_user_token', userId)`
- On future visits, if cookie is missing but localStorage has the token, restore the cookie

**b) Save active writing to localStorage during session:**
- On every keystroke (debounced to 2 seconds), save current text + elapsed time to `localStorage.setItem('anky_draft', JSON.stringify({text, elapsed, timestamp}))`
- Clear the draft on successful `/write` submission

**c) Save anky IDs to localStorage after successful /write:**
- After `/write` returns `anky_id`, save to localStorage

**d) Draft recovery on page load:**
- On page load, check `localStorage.getItem('anky_draft')`
- If a draft exists and is < 1 hour old, offer to restore it into the textarea
- Show a subtle "resume your writing?" prompt

**e) Cookie recovery from localStorage:**
- On page load, if `anky_user_id` cookie is missing but `anky_user_token` exists in localStorage, set the cookie back via JS

### 2. `src/routes/pages.rs` — Pass user token to template

- In the `home()` handler, pass the `user_id` to the template context so the JS can use it as a fallback

### Same persistence for prompt.html flow