feat(sessions): add session browsing for OpenCode and OpenClaw

Add two new session providers following the existing convention-based
pattern. OpenCode reads three-layer JSON storage, OpenClaw parses JSONL
event streams. Wire up backend dispatch, frontend filter dropdown, icon
mappings, toolbar button for OpenClaw, and i18n subtitle updates.

src/components/sessions/utils.ts

@@ -48,6 +48,8 @@ export const getProviderLabel = (
 export const getProviderIconName = (providerId: string) => {
   if (providerId === "codex") return "openai";
   if (providerId === "claude") return "claude";
+  if (providerId === "opencode") return "opencode";
+  if (providerId === "openclaw") return "openclaw";
   return providerId;
 };