fix: sidebar workspace tree + pages DB — emojis → outlined icons

Root cause: 3 sources d'icônes colorées dans le sidebar:
1. _workspace_tree_macro.html L28: {{ page.icon }} raw → filtré via valid_icons
2. db.py: DEFAULT '📄' → 'file'
3. board.py _file_icon(): 15 émojis → 'file'/'edit'/'image'
4. board.py/dashboard.py: JSON icon: 📄/📁 → file/folder
5. base.html L931: JS favorites icon → validNames filter
6. base.html render_tree_item: valid_icons list étendue + fallback

Migration: nouvelles pages utilisent 'file'/'folder'. Pages existantes
avec émojis → les templates utilisent le fallback automatiquement.
143 tests passent

app/routers/dashboard.py

@@ -96,7 +96,7 @@ def _sidebar_data(request: Request, repos: list[dict], include_workspace: bool =
         recent_pages.append({
             "id": full_name,
             "name": repo.get("name", full_name),
-            "icon": "📁",
+            "icon": "folder",
             "url": f"/board/{full_name}",
             "active": False,
             "indent": 0,
@@ -430,7 +430,7 @@ async def help_page(request: Request):
 </div>
 
 <div class="help-card">
-<h3><span class="icon">📁</span>Workspaces</h3>
+<h3><span class="icon">{{ fd_icon("folder",16) }}</span>Workspaces</h3>
 <p>Organize your work into separate workspaces. Each has its own pages and files.</p>
 <ul>
 <li><span class="help-badge local">Local</span> Files stored on your server</li>
@@ -1100,7 +1100,7 @@ def _nav_breadcrumb(conn, page_id: int) -> list:
             "id": row["id"],
             "label": title,
             "url": None,
-            "icon": "\U0001F4C1" if is_folder else _file_icon(title, row["content_format"]),
+            "icon": "folder" if is_folder else _file_icon(title, row["content_format"]),
             "menu": True,
         })
         current = row["parent_id"]
@@ -1160,7 +1160,7 @@ async def nav_menu(request: Request, workspace_id: int = None, parent_id: int =
             items.append({
                 "id": r["id"],
                 "name": title,
-                "icon": "\U0001F4C1" if is_folder else _file_icon(title, r["content_format"]),
+                "icon": "folder" if is_folder else _file_icon(title, r["content_format"]),
                 "has_children": child_counts.get(r["id"], 0) > 0,
                 "url": f"/pages/{r['id']}",
             })