feat(concurrency): provider dropdown instead of free-text input

Replace the Add-form provider text input with a <select> dropdown populated
from the available models' provider prefixes (<provider>/<model>), unioned
with providers already carrying a configured limit (so a limit set out-of-band
but whose model list is empty still appears). The selection auto-defaults to
the first option and falls back when an option disappears.

- logic/view-model.ts: pure providerFromModel + providerOptions(models, limits)
  (distinct provider ids, first-seen order) + 7 tests.
- ConcurrencyView.svelte: models prop + <select> bound to newProviderId;
  disabled + 'No providers available' when there are no models.
- App.svelte: pass models={store.models}; component test updated to the
  dropdown (selectOptions) + a no-models case (6 component tests).

Verified: typecheck 0/0, 922 tests green, biome clean, build OK.

2 files changed, 72 insertions, 0 deletions

diff --git a/src/features/concurrency/logic/view-model.test.ts b/src/features/concurrency/logic/view-model.test.ts
index 336a6eb..82b72b0 100644
--- a/src/features/concurrency/logic/view-model.test.ts
+++ b/src/features/concurrency/logic/view-model.test.ts
@@ -7,6 +7,8 @@ import {
   normalizeConcurrencyStatus,
   parseLimitInput,
   pauseLabel,
+  providerFromModel,
+  providerOptions,
   summarizeLimits,
   summarizeStatus,
   viewConcurrencyLimit,
@@ -44,6 +46,43 @@ describe("parseLimitInput", () => {
   });
 });
 
+// ── providerFromModel / providerOptions ───────────────────────────────────────
+
+describe("providerFromModel", () => {
+  it("takes the part before the first slash", () => {
+    expect(providerFromModel("openai/gpt-4o")).toBe("openai");
+    expect(providerFromModel("openai-compat/gpt-4o-mini")).toBe("openai-compat");
+  });
+  it("returns the whole string when there is no slash", () => {
+    expect(providerFromModel("umans")).toBe("umans");
+  });
+});
+
+describe("providerOptions", () => {
+  it("derives distinct provider ids from models, first-seen order", () => {
+    expect(
+      providerOptions(["openai/gpt-4o", "umans/umans-glm-5.2", "openai/gpt-4o-mini"], []),
+    ).toEqual(["openai", "umans"]);
+  });
+  it("unions with providers already carrying a configured limit", () => {
+    expect(providerOptions(["openai/gpt-4o"], [{ providerId: "anthropic", limit: 4 }])).toEqual([
+      "openai",
+      "anthropic",
+    ]);
+  });
+  it("does not duplicate a provider present in both models and limits", () => {
+    expect(providerOptions(["openai/gpt-4o"], [{ providerId: "openai", limit: 4 }])).toEqual([
+      "openai",
+    ]);
+  });
+  it("ignores models whose provider prefix is empty", () => {
+    expect(providerOptions(["/model-only", "umans/x"], [])).toEqual(["umans"]);
+  });
+  it("returns [] when there are no models and no limits", () => {
+    expect(providerOptions([], [])).toEqual([]);
+  });
+});
+
 // ── pauseLabel + formatPauseDuration ───────────────────────────────────────────
 
 describe("formatPauseDuration", () => {
diff --git a/src/features/concurrency/logic/view-model.ts b/src/features/concurrency/logic/view-model.ts
index 0a4a7a9..7a4fe0e 100644
--- a/src/features/concurrency/logic/view-model.ts
+++ b/src/features/concurrency/logic/view-model.ts
@@ -63,6 +63,39 @@ export function normalizeLimit(value: unknown): number {
   return int >= 1 ? int : 1;
 }
 
+// ── Provider options (the Add-form dropdown) ───────────────────────────────────
+//
+// A concurrency `providerId` is the credential name that prefixes a model name
+// (`<provider>/<model>` — the same key the model picker groups by). The dropdown
+// is the UNION of providers discoverable from the available models AND providers
+// already carrying a configured limit (so a limit set out-of-band but whose
+// model list is empty still appears), in first-seen order. Models are the
+// authority; a provider with models but no limit is still selectable (Add sets it).
+
+/** The provider id prefix of a `<provider>/<model>` name (the part before the first `/`, or the whole string). */
+export function providerFromModel(full: string): string {
+  const i = full.indexOf("/");
+  return i === -1 ? full : full.slice(0, i);
+}
+
+/** Distinct provider ids to offer in the Add dropdown, first-seen order. */
+export function providerOptions(
+  models: readonly string[],
+  limits: readonly ConcurrencyLimitEntry[],
+): string[] {
+  const seen = new Set<string>();
+  const out: string[] = [];
+  const add = (p: string): void => {
+    if (p !== "" && !seen.has(p)) {
+      seen.add(p);
+      out.push(p);
+    }
+  };
+  for (const m of models) add(providerFromModel(m));
+  for (const l of limits) add(l.providerId);
+  return out;
+}
+
 // ── Status → display view ──────────────────────────────────────────────────────
 
 const NO_LIMITS = "No limits configured";