feat: v6.2.0 Web Clipper — extension navigateur (capture article/selection/bookmark/screenshot)

- Service app/services/web_clipper.py: sanitize HTML, html->blocks, extraction article, creation page workspace-aware, rate limit 50/h, device registration
- Router app/routers/web_clipper.py: POST /api/v2/web-clipper/clip, GET /status, POST /auth/verify, GET/DELETE /devices, GET /extensions (download page), auth via session ou Bearer (api_tokens / extension_devices)
- Migration 19: extension_devices + extension_clips (+ indexes)
- Extension Manifest V3: content.js (floating button, selection), background.js (clip + contextMenus), popup.html/js, clipper.css, icons
- Settings UI: onglet Extensions (liste devices, revoke, test clip, liens download), page /extensions
- Tests: 16 tests web_clipper (sanitize, blocks, article/bookmark/selection/screenshot, bearer, rate-limit, devices, extensions page)
- Bump version 6.1.0 -> 6.2.0

static/extension/clipper.css

@@ -0,0 +1,13 @@
+.fd-clipper-btn{
+  position:fixed;bottom:20px;right:20px;z-index:2147483647;
+  background:#2383E2;color:#fff;border:none;border-radius:999px;
+  padding:10px 16px;font-size:13px;font-weight:600;cursor:pointer;
+  box-shadow:0 4px 16px rgba(0,0,0,.25);font-family:system-ui,sans-serif;
+}
+.fd-clipper-btn:hover{background:#1a6fc2}
+.fd-clipper-toast{
+  position:fixed;bottom:60px;right:20px;z-index:2147483647;
+  color:#fff;padding:10px 14px;border-radius:8px;font-size:13px;font-family:system-ui,sans-serif;
+  transform:translateY(10px);opacity:0;transition:all .2s;pointer-events:none;
+}
+.fd-clipper-toast.show{transform:translateY(0);opacity:1}