feat(FN-3062): add /clear command to Chat and Quick Chat, fix session banne

The merge adds a `/clear` command to both the Chat view and Quick Chat that clears transient chat state on session resets, with tests for all new hooks and components. It also persists session banner dismissals via a new `useSessionBannerPref` hook, adds a hide-banner setting, and preserves planning

Fusion-Task-Id: FN-3062

docs/dashboard-guide.md

@@ -35,6 +35,8 @@ Features:
 
 Chat view provides project-scoped conversations with agents.
 
+- Entering `/clear` (exact match after trimming) in the composer starts a fresh thread for the current chat target instead of sending the literal command to the model
+
 ![Chat view](./screenshots/chat-view.png)
 
 ## Quick Chat
@@ -47,6 +49,7 @@ Quick Chat is an optional floating panel for fast, project-scoped assistant conv
 - On small screens, compact tool-call summaries in the floating panel intentionally stay single-line (count + tool names + status) to preserve message density
 - The panel header uses a session-first flow: the main dropdown lists persisted sessions (preferring `session.title`, then falling back to deterministic `Session N` labels)
 - Selecting a session from that dropdown resumes the persisted conversation; this keeps `switchSession()` resume-oriented rather than forcing a new thread
+- Entering `/clear` (exact match after trimming) in the Quick Chat composer uses explicit fresh-session creation for the currently selected target (`startFreshSession()`), so the current thread resets without sending `/clear` to the model
 - The `+` action opens an inline new-session chooser (inside the panel, not a modal) with `Model` selected by default and optional switch to `Agent`
 - Submitting the inline chooser uses explicit fresh-session creation and immediately persists/selects the new thread, then refreshes the session dropdown list
 - Resume lookups still use targeted session queries instead of loading the full active-session list first