a11y pass on viewer UI — keyboard nav, live regions, landmark roles

[lazypower]

Why

PR #5 added the first ARIA affordances to the viewer UI (screen-reader status for the copy action on `MemoryCard.svelte`). A grep across `ui/src` shows that's the only component with any a11y markup at all — the rest of the viewer is mouse-only from the assistive-tech perspective.

```
$ grep -r "aria-|role=|sr-only|tabindex" ui/src
ui/src/components/MemoryCard.svelte: 2 matches (PR #5)
```

Everything else — Header, SearchPanel, TreeBrowser, ProfilePanel, ThemeToggle, App shell — has no semantic landmarks, no keyboard affordances beyond what the browser gives natively, no screen-reader state announcements.

Scope

Per-component audit, then fix in a single PR:

• App.svelte — landmark regions (``, ``, ``), skip-to-content link, heading hierarchy
• Header.svelte — banner role, logo alt text, nav semantics
• ThemeToggle.svelte — `aria-pressed` for toggle state, screen-reader label for "toggle theme"
• SearchPanel.svelte — `role="search"`, labeled input, `aria-live` region for result count / loading state, keyboard submit
• TreeBrowser.svelte — this is the biggest gap. Proper tree semantics (`role="tree"`, `role="treeitem"`, `aria-expanded`, `aria-level`), arrow-key navigation, focus management on expand/collapse
• ProfilePanel.svelte — heading structure, semantic lists
• MemoryCard.svelte — already has copy-state live region; consider whether the card's role="button" is still right when the expanded body is interactive

Non-goals

• Full WCAG 2.1 AA conformance audit — that's a bigger lift (color contrast, motion prefs, etc.). This issue is "bring the rest of the UI up to MemoryCard's baseline."
• Screen-reader testing matrix. We'll verify with VoiceOver + keyboard-only browsing; broader AT coverage can be a follow-up if anyone asks.

Validation

• Keyboard-only walkthrough: Tab through every interactive element; confirm nothing is a keyboard trap and every action is reachable without a mouse.
• VoiceOver rotor: landmarks and headings should give a usable navigation outline.
• `axe-core` / Lighthouse a11y audit in the browser — capture before/after scores in the PR description.

Related

• PR SPA: make memory bodies copyable (follow-up to #1) #5 (the MemoryCard copy affordance that exposed this gap)
• Playwright harness from Add Playwright harness + API-seeded fixtures for UI regression coverage #6 would be a good host for automated a11y smoke tests once it lands.