♻️ refactor(channels): rebuild channel create/edit drawer with modular sections and improved form UX

Restructure the default-theme channel create/edit experience to align with
classic frontend behavior, modern form UX patterns, and the project's Base UI
design system.

Channel editor architecture:
- Split the monolithic channel mutate drawer into focused section components
  (basic, API access, auth, models, advanced) with shared drawer layout
  primitives
- Extract submission, toast handling, and react-query cache invalidation into
  `useChannelMutateForm`
- Add a dedicated loading skeleton for channel detail fetch during edit mode
- Remove the top-level configuration summary block per UX feedback

Form validation and data handling:
- Strengthen `channel-form` Zod schema with JSON, model mapping, status code
  mapping, Codex credential, and Vertex AI key refinements
- Move type-specific conditional validation into `superRefine`
- Normalize base URL formatting and tighten model mapping value validation

Model mapping editor:
- Add Visual/JSON tabbed editing with inline JSON and duplicate-key feedback
- Improve accessibility for icon-only actions and add model suggestion datalists

MultiSelect component:
- Replace the custom cmdk-based implementation with Base UI Combobox chips
- Align focus, border, ring, disabled, and invalid states with standard Input
  styling via `ComboboxChips`
- Preserve existing API (`options`, `selected`, `onChange`, `allowCreate`,
  `createLabel`) for all current callers
- Support inline custom value creation, comma/newline batch input, searchable
  options, portal-based dropdown positioning, and chip removal

Models & groups UX:
- Integrate manual custom model entry directly into the model MultiSelect
- Remove the separate manual model input/button block
- Keep selected-model count and existing model-mapping guardrail behavior

i18n:
- Add and sync translation keys for new editor sections, validation messages,
  model mapping UI, and MultiSelect empty/create labels across en, zh, fr, ja,
  ru, and vi
- Remove obsolete keys tied to the deprecated summary and manual model entry UI

Affected areas:
- `web/default/src/features/channels/components/drawers/`
- `web/default/src/features/channels/hooks/use-channel-mutate-form.ts`
- `web/default/src/features/channels/lib/channel-form.ts`
- `web/default/src/features/channels/lib/model-mapping-validation.ts`
- `web/default/src/features/channels/components/model-mapping-editor.tsx`
- `web/default/src/components/multi-select.tsx`
- `web/default/src/i18n/locales/*.json`

web/default/src/features/channels/lib/model-mapping-validation.ts

@@ -179,6 +179,12 @@ export function validateModelMappingJson(modelMapping: string): {
         error: 'Model mapping must be a valid JSON object',
       }
     }
+    if (Object.values(parsed).some((value) => typeof value !== 'string')) {
+      return {
+        valid: false,
+        error: 'Model mapping values must be strings',
+      }
+    }
     return { valid: true }
   } catch {
     return {