feat: persist user (provider, model) selection per notebook in localStorage

When the user picks a provider+model in the inline prompt, remember
that choice across panel close/reopen AND across JupyterLab restarts,
keyed by notebookPath. If the stored provider (or the model under it)
is no longer present in the current providers payload, fall back to
the default (first provider / default[pid] model) - never silently
apply a stale value that would result in an empty or disabled select.

- New module src/components/model_selection.ts with
  loadModelSelection / saveModelSelection / clearModelSelection.
  Best-effort: localStorage may be disabled (private mode), quota
  may be exhausted, value may be corrupt - all failure modes return
  null / no-op rather than throw.
- IOpenCodeInlinePromptOptions gains notebookPath?: string.
- Constructor resolves the initial selection: prefer stored+valid,
  else first provider / default[pid]. providerSelect.change and
  modelSelect.change listeners call _persistSelection() to keep
  storage in sync.
- _rebuildModelSelect now accepts a preferredModelId; if it exists
  in the current provider's models it's used, else the default.
- OpenCodeCellActions._showPrompt passes notebookPath =
  this._context?.notebookPath so the prompt can key the storage.

Tests
-----
- 10 unit tests in model_selection.spec.ts (load/save/clear +
  malformed JSON + per-notebook independence).
- 4 prompt behavior tests in opencode_cell_actions.spec.ts
  (valid stored applied, provider removed -> default, model removed
  -> default, change -> saved).
- 72 frontend jest (was 58), 52 backend pytest (unchanged),
  build green.

Co-Authored-By: Claude <noreply@anthropic.com>

src/__tests__/opencode_cell_actions.spec.ts

@@ -103,6 +103,12 @@ beforeAll(() => {
   });
 });
 
+beforeEach(() => {
+  // Each test starts with a clean selection store so persistence tests
+  // are deterministic.
+  localStorage.clear();
+});
+
 // Mock the network-touching API module so jsdom tests don't try to hit
 // a real notebook server (and to keep the refresh-history logic decoupled
 // from the network). The cell_actions tests focus on widget behavior.
@@ -323,6 +329,122 @@ describe('OpenCodeCellActions', () => {
     setOpenCodeProviders(null);
   });
 
+  it('restores the user\'s previously-saved (provider, model) when still valid in the providers payload', () => {
+    // Seed localStorage with a prior selection that IS valid.
+    localStorage.setItem(
+      'opencode_bridge:model-selection:foo.ipynb',
+      JSON.stringify({ providerId: 'openai', modelId: 'gpt-5' })
+    );
+    setOpenCodeProviders({
+      providers: [
+        { id: 'anthropic', models: { 'claude-sonnet-4-20250514': { id: 'x' } } },
+        { id: 'openai', models: { 'gpt-5': { id: 'x' }, 'gpt-4': { id: 'x' } } }
+      ],
+      default: { anthropic: 'claude-sonnet-4-20250514' }
+    });
+    const cell = makeFakeCell('x = 1', [], 'foo.ipynb');
+    const actions = new OpenCodeCellActions(cell);
+    Object.defineProperty(actions, 'parent', { value: cell, configurable: true });
+    (actions as any).onAfterAttach({} as any);
+
+    (actions.node.querySelector('button') as HTMLButtonElement).click();
+    const prompt = (actions as any)._prompt;
+    const providerSel = prompt.node.querySelector(
+      '.opencode-provider-select'
+    ) as HTMLSelectElement;
+    const modelSel = prompt.node.querySelector(
+      '.opencode-model-select'
+    ) as HTMLSelectElement;
+    // Stored selection was applied, NOT the default (first provider).
+    expect(providerSel.value).toBe('openai');
+    expect(modelSel.value).toBe('gpt-5');
+    setOpenCodeProviders(null);
+  });
+
+  it('falls back to default when the stored provider is no longer present', () => {
+    localStorage.setItem(
+      'opencode_bridge:model-selection:foo.ipynb',
+      JSON.stringify({ providerId: 'removed-provider', modelId: 'm1' })
+    );
+    setOpenCodeProviders({
+      providers: [
+        { id: 'anthropic', models: { 'claude-sonnet-4-20250514': { id: 'x' } } }
+      ]
+    });
+    const cell = makeFakeCell('x = 1', [], 'foo.ipynb');
+    const actions = new OpenCodeCellActions(cell);
+    Object.defineProperty(actions, 'parent', { value: cell, configurable: true });
+    (actions as any).onAfterAttach({} as any);
+
+    (actions.node.querySelector('button') as HTMLButtonElement).click();
+    const providerSel = (actions as any)._prompt.node.querySelector(
+      '.opencode-provider-select'
+    ) as HTMLSelectElement;
+    // Provider is gone → fall back to first available.
+    expect(providerSel.value).toBe('anthropic');
+    setOpenCodeProviders(null);
+  });
+
+  it('falls back to default model when the stored model is no longer present under the same provider', () => {
+    localStorage.setItem(
+      'opencode_bridge:model-selection:foo.ipynb',
+      JSON.stringify({ providerId: 'openai', modelId: 'gpt-99-removed' })
+    );
+    setOpenCodeProviders({
+      providers: [
+        { id: 'openai', models: { 'gpt-5': { id: 'x' }, 'gpt-4': { id: 'x' } } }
+      ],
+      default: { openai: 'gpt-5' }
+    });
+    const cell = makeFakeCell('x = 1', [], 'foo.ipynb');
+    const actions = new OpenCodeCellActions(cell);
+    Object.defineProperty(actions, 'parent', { value: cell, configurable: true });
+    (actions as any).onAfterAttach({} as any);
+
+    (actions.node.querySelector('button') as HTMLButtonElement).click();
+    const modelSel = (actions as any)._prompt.node.querySelector(
+      '.opencode-model-select'
+    ) as HTMLSelectElement;
+    // Model is gone → fall back to default[openai] = gpt-5.
+    expect(modelSel.value).toBe('gpt-5');
+    setOpenCodeProviders(null);
+  });
+
+  it('saves the new selection to localStorage when the model select changes', () => {
+    setOpenCodeProviders({
+      providers: [
+        { id: 'anthropic', models: { 'claude-sonnet-4-20250514': { id: 'x' } } },
+        { id: 'openai', models: { 'gpt-5': { id: 'x' } } }
+      ]
+    });
+    const cell = makeFakeCell('x = 1', [], 'foo.ipynb');
+    const actions = new OpenCodeCellActions(cell);
+    Object.defineProperty(actions, 'parent', { value: cell, configurable: true });
+    (actions as any).onAfterAttach({} as any);
+
+    (actions.node.querySelector('button') as HTMLButtonElement).click();
+    const prompt = (actions as any)._prompt;
+    const providerSel = prompt.node.querySelector(
+      '.opencode-provider-select'
+    ) as HTMLSelectElement;
+    const modelSel = prompt.node.querySelector(
+      '.opencode-model-select'
+    ) as HTMLSelectElement;
+
+    // Switch provider to openai (whose model list includes gpt-5).
+    providerSel.value = 'openai';
+    providerSel.dispatchEvent(new Event('change'));
+    // Now change the model select to gpt-5.
+    modelSel.value = 'gpt-5';
+    modelSel.dispatchEvent(new Event('change'));
+
+    const stored = JSON.parse(
+      localStorage.getItem('opencode_bridge:model-selection:foo.ipynb') || 'null'
+    );
+    expect(stored).toEqual({ providerId: 'openai', modelId: 'gpt-5' });
+    setOpenCodeProviders(null);
+  });
+
   it('falls back to the first model when default[provider] is not in models', () => {
     setOpenCodeProviders({
       providers: [