Remove WorkspacePanelContent.vue — no production importer

`src/platform/workspace/components/dialogs/settings/WorkspacePanelContent.vue` has no production importer. The only reference in the repo is its own test:

```
src/platform/workspace/components/dialogs/settings/WorkspacePanelContent.test.ts:9
```

The live settings path is `WorkspaceSettingsPanelContent` → `WorkspaceMembersPanelContent` → `MembersPanelContent`. `WorkspacePanelContent` is the older tabbed host; nothing renders it, and its test is what keeps it from being flagged as unused.

### Why it is tracked rather than deleted inline

Raised by @huang47 while reviewing #16553:

> This component has no production importer; only its test references it. If retaining it intentionally, please track deletion separately so this layout-only fix does not leave dead UI indefinitely.

#16553 only adjusts its layout classes, to keep the tabbed host consistent with the live one rather than leave it half-migrated. Deleting it there would have mixed a removal into a layout fix.

### Scope

- Delete `WorkspacePanelContent.vue` and `WorkspacePanelContent.test.ts`.
- Confirm no remaining references (`rg WorkspacePanelContent`), then `pnpm knip` and `pnpm typecheck`.
- Worth a quick check first that nothing intends to revive the tabbed layout — if it is deliberately kept as a staging ground, close this and note that instead.

Start by checking whether the tabbed layout is intentionally being retained, then inspect src/platform/workspace/components/dialogs/settings/WorkspacePanelContent.vue and WorkspacePanelContent.test.ts. Run rg WorkspacePanelContent to confirm there are no remaining references, followed by pnpm knip and pnpm typecheck; done means both files are removed and the checks pass.

Written by the indexing model from the issue text.