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/format.ts

@@ -0,0 +1,9 @@
+/**
+ * Format a token count as a human-readable string (e.g. 1M, 128K, 4096).
+ * Strips trailing ".0" for clean round numbers.
+ */
+export function formatTokenCount(n: number): string {
+  if (n >= 1_000_000) return `${(n / 1_000_000).toFixed(n % 1_000_000 === 0 ? 0 : 1)}M`;
+  if (n >= 1_000) return `${(n / 1_000).toFixed(n % 1_000 === 0 ? 0 : 1)}K`;
+  return String(n);
+}