feat(ai): rendu Markdown, liens fichiers/paths et sessions (#80)

Assistant IA (BooksLM) :

- renderer Markdown complet (titres, listes, tableaux, citations, code) avec echappement HTML

- mentions fichiers/paths cliquables : ouverture du fichier / revelation dans l'arborescence

- gestion multi-sessions par contexte (historique, rechargement, suppression, migration)

- i18n FR/EN, aide in-app, styles, tests frontend (24)

frontend/locales/en.json

@@ -1207,6 +1207,11 @@
   "help.title": "ObsiGate Help",
   "help.title_tags": "Title and tags",
   "help.toc_title": "CONTENTS",
+  "help.assistant_panel": "🧠 Assistant panel (BooksLM)",
+  "help.assistant_panel_desc": "The side assistant (floating button or a folder's context menu) answers in formatted Markdown and contextualises your directories or documents.",
+  "help.assistant_markdown": "Formatted answers: headings, lists, tables, quotes and code blocks.",
+  "help.assistant_links": "Cited files and paths are links: click a file to open it, a folder to reveal it in the tree.",
+  "help.assistant_sessions": "The header history icon lists past sessions (reopen or delete); “+” starts a new conversation.",
   "help.toolbar_section": "AI toolbar",
   "help.tree_section": "Vault tree",
   "help.use_cases": "Use cases",
@@ -1635,6 +1640,11 @@
   "bookslm.no_context": "No files found in this directory",
   "bookslm.context_too_large": "Directory too large — some files were truncated",
   "bookslm.source": "Source",
+  "bookslm.session_history": "Session history",
+  "bookslm.session_delete": "Delete session",
+  "bookslm.history_empty": "No saved session",
+  "bookslm.untitled": "Untitled conversation",
+  "bookslm.no_vault": "No active vault to open this link",
   "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",