feat(models): numbered OpenCode credential selection, labels, migration

Adopt the numbered OPENCODE_API_KEY_N scheme in the model picker and
concurrency surfaces.

- model-select: numeric sort of opencode-<n> keys (#2 before #10); friendly
  OpenCode #N labels (raw opencode-N/<model> value still selected/persisted);
  migrateLegacyModelName rewrites opencode/<model> -> opencode-1/<model>.
- ModelSelector: renders friendly key labels; surfaces a disappeared saved
  credential as UNAVAILABLE (disabled option + notice) without silently
  switching to another model.
- app store: DEFAULT_MODEL -> opencode-1/...; catalogs no longer auto-fallback
  the active model when the selected credential is absent; boot restore
  migrates persisted legacy opencode/<model> tab models idempotently.
- concurrency: providerLabel renders opencode-<n> as OpenCode #<n> in the
  limit rows and Add dropdown (raw provider id preserved on the wire).
- Tests: parsing/sorting/labels/migration, unavailable-state selector behavior,
  no-silent-fallback, and concurrency provider label; fixtures updated to the
  numbered scheme.

Verification: bun install, bun run typecheck (0 errors), bun run test
(1179 passed), bun run check (clean, exit 0), bun run build (vite OK).

1 files changed, 2 insertions, 2 deletions

diff --git a/src/app/App.test.ts b/src/app/App.test.ts
index f43a6b8..06e562f 100644
--- a/src/app/App.test.ts
+++ b/src/app/App.test.ts
@@ -58,7 +58,7 @@ function fakeFetchImpl(): typeof fetch {
   return async (input: string | URL | Request): Promise<Response> => {
     const url = typeof input === "string" ? input : input instanceof URL ? input.href : input.url;
     if (url.endsWith("/models")) {
-      return new Response(JSON.stringify({ models: ["opencode/deepseek-v4-flash"] }), {
+      return new Response(JSON.stringify({ models: ["opencode-1/deepseek-v4-flash"] }), {
         status: 200,
       });
     }
@@ -492,7 +492,7 @@ describe("App component interaction tests", () => {
     const failingFetch = async (input: string | URL | Request): Promise<Response> => {
       const url = typeof input === "string" ? input : input instanceof URL ? input.href : input.url;
       if (url.endsWith("/models")) {
-        return new Response(JSON.stringify({ models: ["opencode/deepseek-v4-flash"] }), {
+        return new Response(JSON.stringify({ models: ["opencode-1/deepseek-v4-flash"] }), {
           status: 200,
         });
       }