[FEATURE]: Model Management UI — custom order, aliases, favorites, and default model

Is your feature request related to a problem? Please describe.

Managing many models across multiple providers is cumbersome in the current UI:

• Models are sorted alphabetically within hardcoded provider groups — users cannot reorder to prioritize frequently used models
• The model name is read-only, taken directly from the provider catalog — users cannot assign friendly aliases (e.g. "My Cheap Fast Model" for a long API ID)
• There is no way to set a default model for new conversations; OpenCode falls back to last-used or an internal priority, which is unpredictable

Describe the solution you'd like

A redesigned model management screen (both dialog-manage-models and settings-models) with four new capabilities:

1. Drag-and-drop reordering

• Allow dragging providers and individual models to set a custom display order
• A toggle in settings to switch between "automatic sort" (current behavior) and "custom order" (user-defined)
• Custom order persisted to opencode.json as a displayOrder array (or equivalent)

2. Inline alias/rename

• Each model shows an editable name field (pencil icon or inline input)
• Changing the name writes the name field to the model's config entry in opencode.json
• The alias is used everywhere: model picker, chat header, system prompt (complementing #36164)

3. Favorite / pin models

• Star/pin icon on models for quick access at the top of the list
• Pinned models appear in a separate "Pinned" section regardless of provider grouping

4. Default model for new conversations

• A "Set as default" button / radio indicator per model
• The default is written to the model field in opencode.json
• New conversations pre-select this model instead of falling back to last-used

// Example resulting config
{
  "model": "anthropic/my-sonnet",              // default model
  "provider": {
    "anthropic": {
      "models": {
        "my-sonnet": {
          "id": "claude-sonnet-4-20250514",
          "name": "Meu Sonnet Padrão"          // alias
        }
      }
    }
  },
  "displayOrder": [                            // custom order
    "anthropic/claude-sonnet-4-20250514",
    "openai/gpt-5",
    "google/gemini-2.5-flash"
  ]
}

Describe alternatives you've considered

• Using only the existing name field in config — works but has no UI, making it inaccessible to non-technical users
• Keyboard shortcut to cycle models — useful but does not solve discoverability or organization

Additional context

This proposal builds on prior discussions:

• #3439 (custom model aliases, completed)
• #36164 (use alias in system prompt, open)
• #23893 (sort providers alphabetically, closed as not planned — a richer proposal may have better acceptance)

The implementation touches:

• packages/app/src/components/dialog-manage-models.tsx — drag handles, rename inputs, pin button, default indicator
• packages/app/src/context/models.tsx — displayOrder, customName, pinned, defaultModel state
• packages/app/src/hooks/use-providers.ts — respect custom order over hardcoded sort
• Persistence layer — write changes back to opencode.json