feat: sidebar user menu Notion-style + settings page + tag colors + preview images/PDF

- Sidebar header: user menu dropdown (Settings, Switch workspace, Log out)
  avec avatar + nom + email, chevron animé, fond opaque #1E1E1E
- Nouvelle page /settings avec gestion globale des tags:
  créer tag avec couleur, changer couleur, supprimer, liste avec compteurs
- API tag management: POST/PUT/DELETE /api/settings/tags, GET /api/settings/tags/all
- Preview panel: affichage images (img tag), PDF (lien viewer), détection format
- workspace-chevron CSS: rotation 180° quand menu ouvert

app/routers/dashboard.py

@@ -1080,6 +1080,80 @@ async def select_workspace(request: Request, ws_id: int):
     return response
 
 
+# ═══════════ Settings Page ═══════════
+
+@router.get("/settings", response_class=HTMLResponse)
+async def app_settings_page(request: Request):
+    """Settings & configuration page."""
+    from jinja2 import Environment, FileSystemLoader
+    env = Environment(loader=FileSystemLoader("app/templates"))
+    sidebar = _sidebar_data(request, [])
+    user = SessionManager.decode_session(request.cookies.get("flowdeck_session", ""))
+    if not user:
+        return RedirectResponse("/auth/login?provider=local", status_code=302)
+    ctx = {**sidebar, "user": user}
+    template = env.get_template("settings.html")
+    return template.render(**ctx)
+
+
+# ═══════════ Tag Management API (global) ═══════════
+
+@router.post("/api/settings/tags")
+async def create_tag_global(request: Request):
+    """Create a tag with optional color."""
+    import json
+    body = await request.json()
+    tag_name = body.get("name", "").strip().lower()
+    color = body.get("color", "#787774")
+    if not tag_name:
+        return {"error": "Tag name required"}, 400
+    with get_conn() as conn:
+        tag = conn.execute("SELECT id FROM tags WHERE name = ?", (tag_name,)).fetchone()
+        if tag:
+            conn.execute("UPDATE tags SET color = ? WHERE id = ?", (color, tag["id"]))
+            conn.commit()
+            return {"tag": {"id": tag["id"], "name": tag_name, "color": color}}
+        cursor = conn.execute("INSERT INTO tags (name, color) VALUES (?, ?)", (tag_name, color))
+        conn.commit()
+        return {"tag": {"id": cursor.lastrowid, "name": tag_name, "color": color}}
+
+
+@router.put("/api/settings/tags/{tag_id:int}")
+async def update_tag_global(tag_id: int, request: Request):
+    """Update a tag (name or color)."""
+    import json
+    body = await request.json()
+    with get_conn() as conn:
+        if "name" in body:
+            conn.execute("UPDATE tags SET name = ? WHERE id = ?", (body["name"].strip().lower(), tag_id))
+        if "color" in body:
+            conn.execute("UPDATE tags SET color = ? WHERE id = ?", (body["color"], tag_id))
+        conn.commit()
+    return {"status": "ok"}
+
+
+@router.delete("/api/settings/tags/{tag_id:int}")
+async def delete_tag_global(tag_id: int):
+    """Delete a tag and its page associations."""
+    with get_conn() as conn:
+        conn.execute("DELETE FROM page_tags WHERE tag_id = ?", (tag_id,))
+        conn.execute("DELETE FROM tags WHERE id = ?", (tag_id,))
+        conn.commit()
+    return {"status": "ok"}
+
+
+@router.get("/api/settings/tags/all")
+async def list_all_tags_global():
+    """List all tags with counts."""
+    with get_conn() as conn:
+        rows = conn.execute(
+            "SELECT t.id, t.name, t.color, COUNT(pt.page_id) as count "
+            "FROM tags t LEFT JOIN page_tags pt ON pt.tag_id = t.id "
+            "GROUP BY t.id ORDER BY t.name"
+        ).fetchall()
+    return {"tags": [dict(r) for r in rows]}
+
+
 # ═══════════ Tags API ═══════════
 
 @router.get("/api/local-workspace/tags")