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, 52 insertions, 6 deletions

diff --git a/src/features/chat/model-select.ts b/src/features/chat/model-select.ts
index 602e0ef..5da6e7b 100644
--- a/src/features/chat/model-select.ts
+++ b/src/features/chat/model-select.ts
@@ -29,15 +29,61 @@ export function joinModelName(key: string, model: string): string {
 /** Distinct keys across all models, in first-seen order. */
 export function modelKeys(models: readonly string[]): string[] {
   const seen = new Set<string>();
-  const out: string[] = [];
+  const openCodeKeys: Array<{ key: string; n: number }> = [];
+  const otherKeys: string[] = [];
   for (const full of models) {
     const { key } = splitModelName(full);
-    if (!seen.has(key)) {
-      seen.add(key);
-      out.push(key);
-    }
+    if (seen.has(key)) continue;
+    seen.add(key);
+    const n = openCodeNumber(key);
+    if (n !== null) openCodeKeys.push({ key, n });
+    else otherKeys.push(key);
   }
-  return out;
+  // Numbered OpenCode credentials sort NUMERICALLY (#2 before #10), placed
+  // before other providers (the primary credential in this app). Others keep
+  // first-seen order.
+  openCodeKeys.sort((a, b) => a.n - b.n);
+  return [...openCodeKeys.map((e) => e.key), ...otherKeys];
+}
+
+/**
+ * The stable numeric identity of a numbered OpenCode provider id/key
+ * (`opencode-1` → 1, `opencode-10` → 10), or `null` when the id is not a
+ * numbered OpenCode key. Numbers are canonical positive integers with no
+ * leading zeros.
+ */
+export function openCodeNumber(key: string): number | null {
+  const m = /^opencode-([1-9][0-9]*)$/.exec(key);
+  return m === null ? null : Number(m[1]);
+}
+
+/** True iff `id` is a numbered OpenCode provider/key (`opencode-<n>`). */
+export function isNumberedOpenCode(id: string): boolean {
+  return openCodeNumber(id) !== null;
+}
+
+/**
+ * A friendly human label for a model key/provider id: `opencode-<n>` →
+ * "OpenCode #<n>"; any other id is returned unchanged. The RAW id is still what
+ * gets selected/persisted — this only affects display.
+ */
+export function modelKeyLabel(key: string): string {
+  const n = openCodeNumber(key);
+  return n === null ? key : `OpenCode #${n}`;
+}
+
+/**
+ * Migration helper: rewrite a legacy `opencode/<model>` full model name to the
+ * numbered `opencode-1/<model>` namespace. Returns the input unchanged for
+ * anything that is not `opencode/`-prefixed (already-numbered names, other
+ * providers). Idempotent. This is the FE-side persisted-state migration; the
+ * backend performs its own equivalent on its own store.
+ */
+export function migrateLegacyModelName(full: string): string {
+  const legacyPrefix = "opencode/";
+  if (!full.startsWith(legacyPrefix)) return full;
+  const model = full.slice(legacyPrefix.length);
+  return model.length === 0 ? full : `opencode-1/${model}`;
 }
 
 /** The model suffixes available under a given key, in order. */