feat(web): add context window support to dashboard config

- Add GET /api/model/info endpoint that resolves model metadata using the
  same 10-step context-length detection chain the agent uses. Returns
  auto-detected context length, config override, effective value, and
  model capabilities (tools, vision, reasoning, max output, model family).

- Surface model.context_length as model_context_length virtual field in
  the config normalize/denormalize cycle. 0 = auto-detect (default),
  positive value overrides. Writing 0 removes context_length from the
  model dict on disk.

- Add ModelInfoCard component showing resolved context window (e.g. '1M
  auto-detected' or '500K override — auto: 1M'), max output tokens, and
  colored capability badges (Tools, Vision, Reasoning, model family).

- Inject ModelInfoCard between model field and context_length override in
  ConfigPage General tab. Card re-fetches on model change and after save.

- Insert model_context_length right after model in CONFIG_SCHEMA ordering
  so the three elements (model input → info card → override) are adjacent.

web/src/lib/api.ts

@@ -43,6 +43,7 @@ export const api = {
   getConfig: () => fetchJSON<Record<string, unknown>>("/api/config"),
   getDefaults: () => fetchJSON<Record<string, unknown>>("/api/config/defaults"),
   getSchema: () => fetchJSON<{ fields: Record<string, unknown>; category_order: string[] }>("/api/config/schema"),
+  getModelInfo: () => fetchJSON<ModelInfoResponse>("/api/model/info"),
   saveConfig: (config: Record<string, unknown>) =>
     fetchJSON<{ ok: boolean }>("/api/config", {
       method: "PUT",
@@ -325,6 +326,24 @@ export interface SessionSearchResponse {
   results: SessionSearchResult[];
 }
 
+// ── Model info types ──────────────────────────────────────────────────
+
+export interface ModelInfoResponse {
+  model: string;
+  provider: string;
+  auto_context_length: number;
+  config_context_length: number;
+  effective_context_length: number;
+  capabilities: {
+    supports_tools?: boolean;
+    supports_vision?: boolean;
+    supports_reasoning?: boolean;
+    context_window?: number;
+    max_output_tokens?: number;
+    model_family?: string;
+  };
+}
+
 // ── OAuth provider types ────────────────────────────────────────────────
 
 export interface OAuthProviderStatus {