feat(mobile): add Scout as an inbox signal source (port #2505)

[Gilbert09]

Summary

Ports #2505 ("feat(signals): add Scouts as a signal source in the inbox") to the mobile app.

Makes the mobile inbox aware of the signals_scout source product (displayed as "Scout", with a Compass icon) so Scout-sourced signal reports display and filter correctly.

Changes (apps/mobile)

• features/inbox/stores/inboxFilterStore.ts — add "signals_scout" to the SourceProduct union.
• features/inbox/components/FilterSheet.tsx — add { value: "signals_scout", label: "Scout" } to the source options.
• features/inbox/components/SignalCard.tsx — add a signals_scout case to the SourceIcon switch (Compass icon, consistent with the other phosphor icons) and render a "Scout · Cross-source issue" / "Scout" label in sourceLine().
• features/inbox/stores/inboxFilterStore.test.ts — test toggling the new source on/off.

Pure renderer/UI surface — no main-process or business-logic changes. Desktop (apps/code) code is untouched.

Testing

• pnpm --filter mobile test src/features/inbox/stores/inboxFilterStore.test.ts passes.
• Biome formatting/lint clean on the changed files.

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
apps/mobile/src/features/inbox/stores/inboxFilterStore.test.ts:18-28
The team prefers parameterized tests (`it.each`). Since the toggle logic is identical for every `SourceProduct` value, expressing this as a table-driven test makes it trivial to extend when new products are added and makes the intent explicit. The two assertions (toggle on / toggle off) still map naturally to two rows.

```suggestion
  it.each<SourceProduct>(["signals_scout", "error_tracking", "github"])(
    "toggles %s in and out of the source filter",
    (source) => {
      const { toggleSourceProduct } = useInboxFilterStore.getState();

      toggleSourceProduct(source);
      expect(useInboxFilterStore.getState().sourceProductFilter).toEqual([
        source,
      ]);

      toggleSourceProduct(source);
      expect(useInboxFilterStore.getState().sourceProductFilter).toEqual([]);
    },
  );
```

_{Reviews (1): Last reviewed commit: "feat(mobile): add Scout as an inbox sign..." | Re-trigger Greptile}

[stamphog]

Review agent failed after 3 attempts — needs human review.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit b4de3ea.}