feat: add global link analytics, like button & live presence (Firestore)

- New link-analytics.js module: click badges, likes, views, live readers
- Floating 👍 like button with spring+fade-out animation on click
- Like button lifted above shared-view pill to avoid overlap
- Inline click count badges on all preview links (global via Firestore FieldValue.increment)
- Analytics panel (slide-in right) with ranked link list + bar chart
- Live 'X reading now' presence via Firestore onSnapshot/heartbeat (20s TTL 60s)
- Document view counter per session for shared docs
- Firestore rules: link_clicks collection + readers subcollection
- Fixed module load order: standalone try/catch after Phase 3a
- Fixed init timing: waitForPreview() poll replaces readyState check

Author: ijbo

changelogs/CHANGELOG-link-analytics.md

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+# Link Analytics — Global Click Tracking, Likes & Live Presence
+
+- Added `js/link-analytics.js` — new module for global cross-user analytics via Firestore
+- Added `css/link-analytics.css` — styles for like widget, click badges, and analytics panel
+- Click count badges appear inline next to each `<a>` link in the preview pane
+- Badge counts are global (shared across ALL users) via Firestore `link_clicks` collection using `FieldValue.increment()`
+- Badges seeded instantly from localStorage display cache; refreshed from Firestore in background
+- Added floating 👍 Like button (fixed, bottom-right) — one like per session, saved to Firestore
+- Like button disappears with a spring-bump → fade-out animation after being clicked
+- Fixed: like button lifted above shared-view pill when `body.shared-view-active` to prevent overlap
+- Fixed: like button lifts above composer FAB on mobile (`bottom: 80px` on ≤768px)
+- Added live "X reading now" presence tracking via Firestore heartbeat/onSnapshot (20s heartbeat, 60s TTL)
+- Added document view counter — increments once per session for shared docs (`link_clicks/views_{docId}`)
+- Added analytics panel (slide-in from right) showing ranked link list with bar chart, opened via toolbar button
+- Panel shows: total opens, live reader count, per-link click count + last clicked date
+- Added `link_clicks` Firestore collection rules (open read/write for analytics, presence subcollection)
+- Fixed: moved `link-analytics.js` out of `Promise.all` Phase 3a into its own `try/catch` await after Phase 3a
+- Fixed: replaced fragile `readyState` bootstrap with `waitForPreview()` poll (100ms × 100 retries) to handle async module load order
+- Updated `src/main.js` to import `link-analytics.css` and load `link-analytics.js`
+- Updated `firestore.rules` to allow `link_clicks` collection and `readers` subcollection
+
+---
+
+## Summary
+Implements a complete global analytics system for TextAgent shared documents. All metrics (link clicks, likes, document views, live readers) are stored in Firestore and shared across every user — not scoped to a single browser session.
+
+---
+
+## 1. Global Link Click Tracking
+**Files:** `js/link-analytics.js`, `css/link-analytics.css`
+**What:** Intercepts clicks on all `<a>` links in the markdown preview. Each click atomically increments a Firestore counter (`FieldValue.increment(1)`) under `link_clicks/{urlSHA256}`. A small pill badge renders inline after the link text showing the global click count.
+**Impact:** Any user clicking a link in any TextAgent document contributes to a shared click count visible to everyone.
+
+## 2. Floating Like Button
+**Files:** `js/link-analytics.js`, `css/link-analytics.css`
+**What:** Injects a fixed-position 👍 button (bottom-right) into the preview pane. One like per browser session (memory-only dedup). Saves to `link_clicks/likes_{shareDocId}` with Firestore realtime listener for live count. On click: spring animation → fade-out → `display:none`.
+**Impact:** Readers of shared documents can express appreciation. Like count updates in realtime across all viewers.
+
+## 3. Live Presence (Reading Now)
+**Files:** `js/link-analytics.js`
+**What:** When a shared doc is opened, writes a heartbeat document to `link_clicks/presence_{docId}/readers/{sessionId}` every 20s. A Firestore `onSnapshot` listener counts sessions with `lastSeen` within 60s and updates the green pulsing "X reading now" pill in the analytics panel header.
+**Impact:** Shows how many people are actively reading a shared document right now, in realtime.
+
+## 4. Document View Counter
+**Files:** `js/link-analytics.js`
+**What:** On first load of a shared doc per session, increments `link_clicks/views_{docId}.views` in Firestore. Displayed in the analytics panel as "N total opens".
+**Impact:** Authors can see total lifetime opens of their shared documents.
+
+## 5. Analytics Panel
+**Files:** `js/link-analytics.js`, `css/link-analytics.css`
+**What:** Slide-in right panel (420px wide, keyboard-dismissable) opened via a bar chart toolbar button. Fetches all click data from Firestore, renders a ranked list with proportional bars (gold/silver/bronze for top 3), total opens, and live reader count.
+**Impact:** Central dashboard for all link and document engagement metrics.
+
+## 6. Firestore Rules
+**Files:** `firestore.rules`
+**What:** Added `link_clicks/{docId}` collection rules (open read/write) and `readers/{sessionId}` subcollection rules (write limited to `{lastSeen: int}` shape, delete allowed for cleanup).
+**Impact:** Enables anonymous read/write for analytics without touching the protected `shares` collection rules.
+
+## 7. Module Load Fix
+**Files:** `src/main.js`, `js/link-analytics.js`
+**What:** Extracted from `Promise.all` Phase 3a into a standalone `try/catch await` after Phase 3a. Replaced `readyState` bootstrap with `waitForPreview()` — a 100ms polling loop that waits up to 10s for `M.markdownPreview`, `M.renderMarkdown`, and `M.db` to be available.
+**Impact:** Eliminates silent initialization failures when the module loads before the preview element is wired.
+
+---
+
+## Files Changed (4 total)
+
+| File | Type | Description |
+|------|------|-------------|
+| `js/link-analytics.js` | NEW | Core analytics module (~570 lines) |
+| `css/link-analytics.css` | NEW | All styles for badges, like widget, panel |
+| `src/main.js` | MODIFIED | CSS import + standalone module load |
+| `firestore.rules` | MODIFIED | `link_clicks` collection + presence rules |