test(v2): add vitest and cargo tests for sdk-messages, agent-tree, session, ctx

Frontend (vitest):
- sdk-messages.test.ts: adaptSDKMessage() for all 9 message types
- agent-tree.test.ts: buildAgentTree(), countTreeNodes(), subtreeCost()
- vite.config.ts: vitest test config (src/**/*.test.ts)
- package.json: vitest ^4.0.18 dev dep, "test" script

Backend (cargo):
- session.rs: SessionDb CRUD tests (sessions, SSH, settings, layout) with tempfile
- ctx.rs: CtxDb error handling tests with missing database
- Cargo.toml: tempfile 3 dev dependency

v2/src-tauri/src/ctx.rs

@@ -170,3 +170,68 @@ impl CtxDb {
         Ok(entries)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Create a CtxDb with conn set to None, simulating a missing database.
+    fn make_missing_db() -> CtxDb {
+        CtxDb { conn: Mutex::new(None) }
+    }
+
+    #[test]
+    fn test_new_does_not_panic() {
+        // CtxDb::new() should never panic even if ~/.claude-context/context.db
+        // doesn't exist — it just stores None for the connection.
+        let _db = CtxDb::new();
+    }
+
+    #[test]
+    fn test_list_projects_missing_db_returns_error() {
+        let db = make_missing_db();
+        let result = db.list_projects();
+        assert!(result.is_err());
+        assert_eq!(result.unwrap_err(), "ctx database not found");
+    }
+
+    #[test]
+    fn test_get_context_missing_db_returns_error() {
+        let db = make_missing_db();
+        let result = db.get_context("any-project");
+        assert!(result.is_err());
+        assert_eq!(result.unwrap_err(), "ctx database not found");
+    }
+
+    #[test]
+    fn test_get_shared_missing_db_returns_error() {
+        let db = make_missing_db();
+        let result = db.get_shared();
+        assert!(result.is_err());
+        assert_eq!(result.unwrap_err(), "ctx database not found");
+    }
+
+    #[test]
+    fn test_get_summaries_missing_db_returns_error() {
+        let db = make_missing_db();
+        let result = db.get_summaries("any-project", 10);
+        assert!(result.is_err());
+        assert_eq!(result.unwrap_err(), "ctx database not found");
+    }
+
+    #[test]
+    fn test_search_missing_db_returns_error() {
+        let db = make_missing_db();
+        let result = db.search("anything");
+        assert!(result.is_err());
+        assert_eq!(result.unwrap_err(), "ctx database not found");
+    }
+
+    #[test]
+    fn test_search_empty_query_missing_db_returns_error() {
+        let db = make_missing_db();
+        let result = db.search("");
+        assert!(result.is_err());
+        assert_eq!(result.unwrap_err(), "ctx database not found");
+    }
+}