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.

backend/bookslm_routes.py

@@ -53,6 +53,11 @@ class BooksLMContextRequest(BaseModel):
         default_factory=list,
         description="Ad-hoc directories added with the '@' command (any mode)",
     )
+    app_context: dict[str, Any] | None = Field(
+        default=None,
+        description="Live client UI state for the General assistant: open_documents, "
+                    "current_path, directory, vault, search_query, search_total, search_results.",
+    )
 
 
 class BooksLMChatRequest(BaseModel):
@@ -104,6 +109,11 @@ class BooksLMChatRequest(BaseModel):
         description="Conversation snapshot returned alongside a ``confirmation`` event, "
                     "echoed back to resume the agent run.",
     )
+    app_context: dict[str, Any] | None = Field(
+        default=None,
+        description="Live client UI state for the General assistant: open_documents, "
+                    "current_path, directory, vault, search_query, search_total, search_results.",
+    )
 
 
 def _normalize_mode(mode: str | None) -> str:
@@ -154,6 +164,34 @@ def _build_context(
     return base
 
 
+def _submitted_app_context(req) -> dict[str, Any] | None:
+    """Return the client-submitted live app state (may be None)."""
+    ctx = getattr(req, "app_context", None)
+    return ctx if isinstance(ctx, dict) else None
+
+
+def _recent_files_for_prompt(current_user, limit: int = 10) -> list[dict[str, Any]]:
+    """Best-effort list of the user's most recently modified files.
+
+    Used to give the General assistant a sense of what the user has been
+    working on. Never raises: any failure yields an empty list.
+    """
+    try:
+        from backend.services.recent import list_recent
+
+        username = current_user.get("username") if isinstance(current_user, dict) else None
+        user_vaults = (
+            current_user.get("_token_vaults") or current_user.get("vaults", [])
+            if isinstance(current_user, dict)
+            else []
+        )
+        data = list_recent(username, user_vaults, limit=limit, mode="modified")
+        return list(data.get("files", []))
+    except Exception:  # pragma: no cover - defensive, prompt enrichment only
+        logger.debug("Could not gather recent files for assistant prompt", exc_info=True)
+        return []
+
+
 def _resolve_system_prompt(req, current_user) -> str:
     """Resolve the vault access and build the assistant system prompt.
 
@@ -179,11 +217,28 @@ def _resolve_system_prompt(req, current_user) -> str:
     effective_mode = context.get("scope", mode)
 
     if effective_mode == "general":
-        prompt = build_general_system_prompt(list(index.keys()))
+        prompt = build_general_system_prompt(
+            list(index.keys()),
+            app_context=_submitted_app_context(req),
+            recent_files=_recent_files_for_prompt(current_user),
+        )
     elif effective_mode == "documents":
         prompt = build_system_prompt(context, scope="documents")
     elif context["file_count"] == 0:
-        raise HTTPException(status_code=404, detail="Aucun fichier markdown trouvé dans ce dossier")
+        # Empty (or unreadable) directory: don't block the request. Answer as
+        # the General assistant would, telling the model the folder is empty so
+        # it can still help (create a file, explain the app, etc.).
+        prompt = build_general_system_prompt(
+            list(index.keys()),
+            app_context=_submitted_app_context(req),
+            recent_files=_recent_files_for_prompt(current_user),
+        )
+        prompt += (
+            f"\n## Dossier vide\nLe dossier « {req.directory or '/'} » "
+            f"(vault {req.vault}) ne contient aucun fichier markdown exploitable. "
+            "Réponds quand même à la demande de l'utilisateur sans contexte "
+            "documentaire, et propose une action de création si c'est pertinent.\n"
+        )
     else:
         prompt = build_system_prompt(context, scope="directory")