fix(ai): contexte applicatif de l'assistant, dossier vide et liens fiables (#88, BUG-041, BUG-042)

- BUG-041 : le contexte d'un dossier vide ne renvoie plus 404 ; il degrade
  vers le prompt General enrichi d'un bloc « Dossier vide ».
- BUG-042 : liens de fichiers deterministes (nom -> presse-papiers,
  dossier -> arborescence, chemin -> viewer) et resolution du chemin contre
  l'index du vault (exact -> suffixe -> basename) avant ouverture, fin des
  'File not found'.
- #88 : app_context (documents ouverts, repertoire/vault courants, recherche
  + resultats affiches) et fichiers recemment modifies injectes dans le
  prompt General.

Tests : pytest 963 passed / 6 skipped, ruff 0, mypy 0, frontend 52/52 +
validate-imports 36 modules.

frontend/locales/en.json

@@ -1271,9 +1271,9 @@
   "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_panel_desc": "The side assistant (floating button or a folder's context menu) answers in formatted Markdown and contextualises your directories or documents. In general mode it also knows what you are looking at: open documents, current directory, active search and recently modified files.",
   "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_links": "Cited files and paths are links: a bare filename copies the name to the clipboard, a folder is revealed in the tree, and a file path opens it in the viewer.",
   "help.assistant_sessions": "The header history icon lists past sessions (reopen or delete); “+” starts a new conversation.",
   "help.assistant_agent": "The \"agent mode\" button enables tools (read, list, search); modifying actions require confirmation with a change preview.",
   "help.assistant_resize": "The left edge of the panel is resizable; the width is remembered.",
@@ -1771,6 +1771,7 @@
   "bookslm.history_empty": "No saved session",
   "bookslm.untitled": "Untitled conversation",
   "bookslm.no_vault": "No active vault to open this link",
+  "bookslm.copied_name": "Name copied: {name}",
   "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",