app: expose server's default context length to UI (#14037)

Parse the default_num_ctx from the server's "vram-based default context"
log line and expose it through the inference compute API. This eliminates
duplicate VRAM tier calculation logic in the frontend.

- Add InferenceInfo struct with Computes and DefaultContextLength
- Rename GetInferenceComputer to GetInferenceInfo
- Handle missing default context line gracefully (older servers)
- Add DefaultContextLength to InferenceComputeResponse
- Update Settings UI to use server's default, disable slider while loading
- Add disabled prop to Slider component (grays out + hides handle)
- Migrate existing users with context_length=4096 to 0 (auto mode)

app/store/testdata/schema.sql

@@ -13,7 +13,7 @@ CREATE TABLE IF NOT EXISTS settings (
     agent BOOLEAN NOT NULL DEFAULT 0,
     tools BOOLEAN NOT NULL DEFAULT 0,
     working_dir TEXT NOT NULL DEFAULT '',
-    context_length INTEGER NOT NULL DEFAULT 4096,
+    context_length INTEGER NOT NULL DEFAULT 0,
     window_width INTEGER NOT NULL DEFAULT 0,
     window_height INTEGER NOT NULL DEFAULT 0,
     config_migrated BOOLEAN NOT NULL DEFAULT 0,