feat(ai): disable local provider and improve commit file display

Local AI is now treated as unavailable in the settings UI, with a
migration to ensure any previously saved "local" selection switches
back to OpenAI. The history panel also improves how commit file
names are presented, including clearer old->new path formatting.

- Migrate stored AI provider away from local to prevent dead state
- Disable local provider option with an "in development" badge
- Refine history panel filename rendering and tooltip context

src/App.svelte

@@ -543,7 +543,7 @@
 
   function defaultAiSettings(): AiSettings {
     return {
-      provider: "local",
+      provider: "openai",
       localModelId: "qwen2.5-0.5b",
       localProfile: "fast",
       openaiModel: "gpt-4o-mini",
@@ -557,7 +557,11 @@
     try {
       const stored = JSON.parse(localStorage.getItem(AI_SETTINGS_KEY) ?? "null") as unknown;
       if (stored && typeof stored === "object") {
-        return { ...defaultAiSettings(), ...(stored as Partial<AiSettings>) };
+        const merged = { ...defaultAiSettings(), ...(stored as Partial<AiSettings>) };
+        // Local AI is still in development and disabled in the settings UI — migrate any
+        // previously saved selection away from it so nobody gets stuck on a dead option.
+        if (merged.provider === "local") merged.provider = "openai";
+        return merged;
       }
     } catch {
       // Fall through to defaults below.