feat(ai): assistant AI multi-contexte + ergonomie sidebar et clavier

Contexte:
- Repertoire (mode historique BooksLM, via menu contextuel dossier)
- Documents (bouton flottant quand des documents sont ouverts)
- General (bouton flottant sans document: aide app + creation de fichiers)
- Le contexte est affiche dans le header; suggestions adaptees au mode
- Blocs obsigate-action rendus en carte avec bouton Appliquer (pas d'execution auto)

Backend:
- /api/ai/bookslm/context et /chat acceptent mode + context_files
- collect_files_context(), empty_context(), build_general_system_prompt()
- mode documents degrade vers general si aucun fichier lisible

Corrige:
- bouton flottant passait le chemin d'un fichier comme repertoire -> erreur
  'Aucun fichier markdown trouve dans ce dossier'
- panneau precedent masque (localStorage) ne se rouvrait plus
- selecteur fournisseur/modele deplace sur une ligne dediee (header actions visibles)
- Entree envoie, Ctrl+Entree insere un saut de ligne

Tests: test_bookslm.py (+13) et tests/frontend/ai.test.mjs (+8)

frontend/locales/en.json

@@ -78,8 +78,20 @@
   "admin.users_manage_btn": "Manage users",
   "admin.users_manage_hint": "Open the user management modal (full CRUD)",
   "admin.users_title": "User management",
+  "ai.action_applied": "Applied ✓",
+  "ai.action_apply": "Apply",
+  "ai.action_applying": "Applying…",
+  "ai.action_create_dir": "Create folder {path}",
+  "ai.action_create_file": "Create file {path}",
+  "ai.action_created_dir": "Folder created: {path}",
+  "ai.action_created_file": "File created: {path}",
+  "ai.action_failed": "Action failed: {error}",
   "ai.casual": "Casual tone",
+  "ai.close": "Close assistant",
   "ai.completion_added": "AI: completion added",
+  "ai.context_documents": "{count} open document(s)",
+  "ai.context_general": "General context",
+  "ai.context_general_desc": "Questions about the app and file creation",
   "ai.continue": "Continue",
   "ai.custom_rewrite": "Custom rewrite",
   "ai.done": "AI: text processed",
@@ -89,6 +101,7 @@
   "ai.fix_spelling": "Fix spelling",
   "ai.fix_spelling_hint": "Fixes spelling and grammar",
   "ai.frontmatter": "Generate frontmatter",
+  "ai.fullscreen": "Fullscreen",
   "ai.generate": "Generate",
   "ai.hint_add_details": "Adds details",
   "ai.hint_edit_selection": "Modifies the selected text",
@@ -96,6 +109,7 @@
   "ai.hint_improve": "Improves text quality",
   "ai.hint_translate": "Translates the selected text",
   "ai.improve": "Improve",
+  "ai.input_hint": "Enter to send · Ctrl+Enter for a line break",
   "ai.inline_completion_title": "AI Inline Completion (Ctrl+J) — Auto-complete text",
   "ai.invalid_api_key": "AI: invalid API key. Check DEEPSEEK_API_KEY in .env",
   "ai.longer": "Longer",
@@ -110,6 +124,7 @@
   "ai.model_loading": "Loading…",
   "ai.model_offline": "offline",
   "ai.model_load_error": "Load error",
+  "ai.model_settings": "Provider & model",
   "ai.quota_exceeded": "AI: quota exceeded or payment required",
   "ai.rewrite": "💬 Rewrite",
   "ai.rewrite_done": "AI: text rewritten",
@@ -121,6 +136,12 @@
   "ai.shorter_hint": "Makes the text more concise",
   "ai.simplify": "Simplify",
   "ai.simplify_hint": "Simplifies the language",
+  "ai.suggestion_capabilities": "What can you do?",
+  "ai.suggestion_create_file": "Create a meeting notes file",
+  "ai.suggestion_docs_contradictions": "Are there contradictions between these documents?",
+  "ai.suggestion_docs_keypoints": "What are the key points of these documents?",
+  "ai.suggestion_docs_summary": "Summarize the open documents",
+  "ai.suggestion_search": "How do I search my notes effectively?",
   "ai.summarize": "Summarize",
   "ai.text_processed": "AI: text processed ✓",
   "ai.text_rewritten": "AI: text rewritten",
@@ -1614,7 +1635,9 @@
   "bookslm.context_too_large": "Directory too large — some files were truncated",
   "bookslm.source": "Source",
   "palette.bookslm_open": "BooksLM: Open for current directory",
+  "palette.bookslm_open_desc": "Opens the AI assistant for the current context (open documents or general)",
   "palette.bookslm_new": "BooksLM: New conversation",
+  "palette.bookslm_new_desc": "Resets the AI assistant conversation",
   "fab.open": "Open AI assistant",
   "fab.close": "Close AI assistant",
   "mfa.webauthn_title": "Security keys (WebAuthn)",