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/db.py

@@ -192,7 +192,7 @@ def init_db():
                 id INTEGER PRIMARY KEY AUTOINCREMENT,
                 collection_id INTEGER NOT NULL REFERENCES collections(id) ON DELETE CASCADE,
                 title TEXT NOT NULL DEFAULT '',
-                icon TEXT DEFAULT '📄',
+                icon TEXT DEFAULT 'file',
                 position INTEGER NOT NULL DEFAULT 0,
                 parent_id INTEGER REFERENCES collection_pages(id),
                 gitea_issue_id INTEGER,