Mobile: solo-architect tab label regresses from 'Architect' to 'main' after #762 #764

Symptom

For solo-architect users on mobile, the TabBar tab labeled `Architect` is now labeled `main` after PR #762 merged.

Why

PR #762's `useTabs.ts` changed the architect-tab `label` from the hardcoded string `'Architect'` to the architect's `name`. For an N=1 workspace, the default name is `main`, so the label silently changes.

```diff
-tabs.push({ id: 'architect', label: 'Architect', ... });
+label: name, // = 'main' for the default architect
```

Caught by Codex's spec review (HIGH REQUEST_CHANGES) before merge but not blocking — the regression affects only mobile + solo-architect users (tiny population) and is purely cosmetic.

Fix shape (~5 lines)

When `architects.length === 1` (or for the first/bare architect tab whenever the rest of the UI doesn't visibly differentiate), use `'Architect'` as the label. When there are siblings, use the architect name.

Concretely in `buildArchitectTabs`:

```ts
label: architects.length === 1 ? 'Architect' : name,
```

Plus a test asserting N=1 label is `'Architect'`.

Suggested protocol

BUGFIX — isolated 5-line change in one file + 1 test.