feat(sessions): add stop confirmation, health checks, and tunnel recreation

- Add inline confirmation dialog before stopping instances
- Delete instances from state immediately without page reload
- Add health check polling every 30s for running instances
- Show tunnel error badge when tunnel is unreachable
- Add 'Fix Tunnel' button to recreate broken tunnels
- Update API client with health check and tunnel recreation endpoints

openspec/changes/session-management-fixes/specs/tunnel-health-monitoring/spec.md

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+## ADDED Requirements
+
+### Requirement: System monitors tunnel health
+The system SHALL periodically check if active tunnel URLs are reachable and mark them as erroneous if not.
+
+#### Scenario: Healthy tunnel
+- **WHEN** a tunnel health check is performed on a running instance
+- **THEN** the system receives an HTTP 2xx response
+- **AND** the instance status remains "running"
+
+#### Scenario: Broken tunnel
+- **WHEN** a tunnel health check is performed on a running instance
+- **AND** the response is not HTTP 2xx or the request fails
+- **THEN** the instance is marked with tunnel_error status
+- **AND** a visual error indicator is displayed in the UI
+
+### Requirement: Users can recreate broken tunnels
+The system SHALL allow users to regenerate a temporary tunnel for a running instance without restarting the instance.
+
+#### Scenario: Recreate tunnel
+- **WHEN** user clicks "Recreate Tunnel" button on an instance with a broken tunnel
+- **THEN** the system stops the existing cloudflared process
+- **AND** starts a new cloudflared tunnel
+- **AND** updates the instance URL
+- **AND** the new URL is displayed in the UI
+
+#### Scenario: Recreate tunnel success
+- **WHEN** tunnel recreation completes successfully
+- **THEN** the error indicator is removed
+- **AND** the instance shows as healthy
+
+#### Scenario: Recreate tunnel failure
+- **WHEN** tunnel recreation fails
+- **THEN** the error indicator remains
+- **AND** an error message is displayed to the user