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/templates/_workspace_tree_macro.html

@@ -1,3 +1,4 @@
+{% from '_icons.html' import fd_icon %}
 {% macro render_workspace_tree(pages, depth=0) %}
   {% for page in pages %}
   {% set safe_name = page.name | replace("'", "\\'") %}
@@ -25,7 +26,7 @@
     {% else %}
     <span style="width:12px;flex-shrink:0;"></span>
     {% endif %}
-    <span class="page-icon">{{ page.icon }}</span>
+    <span class="page-icon">{% set valid_icons = ["folder","file","star","link","trash","book","home","settings","lock","globe","image","edit","calendar","users","user","search","tag","bar-chart","grid","list","align-left","zap"] %}{% if page.icon in valid_icons %}{{ fd_icon(page.icon,14) }}{% elif page.is_folder %}{{ fd_icon("folder",14) }}{% else %}{{ fd_icon("file",14) }}{% endif %}</span>
     {% if page.is_folder %}
     <span class="page-name tree-folder-link" 
           @click.stop="navigateToFolder({{ page.db_id }})"