feat(ai-assistant): make the AI-disabled error link to AI Settings

[epeicher]

What & why

When AI features aren't enabled, the AI Assistant showed a plain red sentence — "AI features are not enabled. Enable them in OS Settings → AI Settings." — with nothing to click. This makes the "OS Settings → AI Settings" phrase an inline link that opens OS Settings directly on the AI tab, so the error is recoverable in one click.

Changes

AI Assistant (src/ai-assistant/impl.ts)

• On the desktop_mode_ai_disabled error, linkify the "OS Settings → AI Settings" phrase. Keyed on the server error code, not the wording, so copy/translation tweaks don't break the affordance; falls back to a trailing link if the phrase is absent.
• New _openAiSettings() closes the assistant and deep-links to the AI tab.
• Threaded the error code through the error paths that carry it (fetch 403 body + SSE error event).

openOsSettings({ tabId }) (src/desktop.ts, src/settings/index.ts, src/api/facade.ts)

• openOsSettings now accepts an optional { tabId }. The tab is set before the window opens (fresh mount lands on it); new OsSettings.focusTab() live-switches the <wpd-tabs> value when the window is already open.
• Backwards-compatible, additive widening of the public type. Also makes the previously dead tab-targeting callers work (item-visibility-menu already passed { tabId: 'apps-icons' }); fixed plugins-window which passed a bare string.

Styling (assets/css/ai-assistant.css, assets/css/desktop.css)

• Style the phrase as an inline text link (bold + underline, inherits the error color).
• Add a scoped pointer-cursor exception to the shell's deliberate cursor: default !important no-pointer policy, so a genuine link reads as clickable. Scoped under body.desktop-mode-active to out-specify the policy (same trick the resize-handle exceptions use), and cross-referenced in the policy comment so the documented exception list stays accurate.

Caching (includes/assets.php)

• filemtime-stamp ai-assistant.css (was on the static plugin version) so this fast-iterating surface busts cache without a version bump — same pattern windows.css/chromeless.css already use.

Docs

• Documented openOsSettings({ tabId }) in javascript-reference.md and dock-customization.md.

Test plan

1. As a user with AI features disabled, open the AI Assistant and submit any query.
2. The error now renders "…Enable them in OS Settings → AI Settings." with the phrase as an underlined link (pointer cursor on hover).
3. Clicking it closes the assistant and opens OS Settings on the AI tab.
4. Bonus, now working: right-click a dock/desktop tile → Apps & Icons settings… lands on the Apps & Icons tab; the Plugins window "configure features" entry lands on the Features tab.

Gates: typecheck, lint, test:js (1519 tests) green; full build clean.

Note

The opt-in SSE transport returns a bare 403 before streaming on this gate, so that (non-default) path still shows the generic "Lost connection" message rather than the link. Left out of scope; happy to make the SSE handler emit the disabled error too if wanted.