Configurable session title language + per-session summary tooltip for easier session discovery

Problem

I frequently have to start a new session for each task because long-running sessions get unwieldy. Over time this creates a large pile of sessions, and:

1. Old sessions are hard to find — the auto-generated title is the only signal, and it doesn't always capture what the session was actually about.
2. Titles aren't in my preferred language — there's currently no way to configure the language of the auto-generated title. It mostly follows the conversation, but not reliably, and there's no setting to force it.
3. No knowledge carries across sessions — each session is isolated. When I hit the same problem again in a future session, the assistant repeats the same mistakes it made before, because there's no summary/continuity bridging sessions.

Requested features

1. Configurable session-title language — a setting (e.g. in settings.json) to force the auto-generated session title into a chosen language, instead of leaving it to the summarizer.
2. Per-session summary with hover tooltip — generate a short summary for each session and show it when hovering over the session title in the sidebar, so I can quickly identify the right past session without opening each one.
3. (Root cause) Cross-session memory/continuity — a lightweight summary that persists between sessions so the assistant doesn't repeat earlier mistakes when the same kind of problem recurs.

Why

The underlying pain is session discoverability and continuity: lots of short single-task sessions, hard to locate later, with no shared learning between them. Better titles + hover summaries solve discovery; cross-session summaries solve the "AI makes the same mistake again" problem.