fix(a11y): add aria-label to mobile tab selector in PageTabBar (#1871)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - People increasingly drive Paperclip from a phone, so the UI ships a
mobile layout alongside the desktop one
> - `PageTabBar` is the shared component behind the tab strip on nearly
every detail page — AgentDetail, ProjectDetail, RoutineDetail,
IssueDetail, Inbox, Costs
> - On desktop it renders a Radix `TabsList`, whose `TabsTrigger`s carry
their own accessible names; on mobile it swaps to a native `<select>`
> - That `<select>` had no accessible name at all, so screen readers
announced it only as "popup button" — a user could not tell what the
control switches between
> - Because the component is shared, the gap reproduced on every mobile
page that uses tabs rather than on one screen
> - This pull request adds `aria-label="Page section"` to the mobile
`<select>`
> - The benefit is that mobile screen-reader users get the same
orientation desktop users already get from the tab triggers, from a
one-line change with no visual or behavioral impact

## Linked Issues or Issue Description

No existing public issue covers this, so the problem is described in-PR
following `.github/ISSUE_TEMPLATE/bug_report.yml`:

- **What happened** — On a mobile viewport, the `PageTabBar` `<select>`
had no `aria-label`, no `<label>` association, and no visible text of
its own. VoiceOver/TalkBack announce it as an unlabeled "popup button".
- **Expected behavior** — The control announces what it switches
between, matching the accessible naming the desktop `TabsTrigger`s
already provide.
- **Steps to reproduce** — 1. Open any detail page with tabs (agent,
project, routine, issue). 2. Narrow the viewport to mobile width so the
tab strip collapses to a `<select>`. 3. Focus the `<select>` with a
screen reader. 4. Observe that no purpose is announced.
- **Version / commit** — head `ef92d1c`, branch
`fix/page-tab-bar-mobile-a11y`.
- **Deployment mode** — Any. The change is UI-only and client-side.

**Related prior PR:** #1532 (closed unmerged on 2026-03-23) made this
same one-line change to `ui/src/components/PageTabBar.tsx` as part of a
~100-file batch. This PR is the focused standalone version of that fix.

## What Changed

- Added `aria-label="Page section"` to the mobile `<select>` in
`ui/src/components/PageTabBar.tsx`. One line added; no other files
touched.

## Verification

- **Automated:** `pnpm -C ui test` and the repo CI gates (lint,
typecheck, build) — CI is currently green on `ef92d1c`.
- **Manual:** Open any tabbed detail page, narrow the viewport until the
tab strip becomes a `<select>`, and focus it with VoiceOver (macOS/iOS)
or TalkBack (Android). It now announces "Page section, popup button"
instead of an unlabeled "popup button".
- **Inspector check:** In devtools, the `<select>` node's computed
accessible name is "Page section" (previously empty).

## Risks

Low risk. `aria-label` on a `<select>` is a presentation-free attribute:
it changes nothing about layout, styling, DOM structure, event handling,
or the desktop code path, which is untouched. No migration, no API
change, no new dependency. The only debatable point is wording — "Page
section" is a generic name shared by every call-site (see the note
below).

## Model Used

Not specified by the original author, and not recoverable from the
commit metadata (no `Co-Authored-By` or model trailer on `ef92d1c`).
@bluzername — please replace this line with the provider, model
ID/version, and any relevant capability details, or "None —
human-authored".

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [ ] I have specified the model used (with version and capability
details) — see above; needs the author
- [ ] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change
(`fix/page-tab-bar-mobile-a11y`) and contains no internal Paperclip
ticket id or instance-derived details
- [ ] I have run tests locally and they pass
- [ ] I have added or updated tests where applicable — no test added;
the change is a static attribute with no branching behavior
- [ ] I have updated relevant documentation to reflect my changes — not
applicable
- [x] I have considered and documented any risks above
- [x] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups —
currently 4/5, see below
- [ ] I will address all Greptile and reviewer comments before
requesting merge

---

### Maintainer note on the open Greptile comment

This description was restructured to the repository PR template by a
maintainer; the code and the author's intent are unchanged. Unchecked
boxes above are ones only @bluzername can attest to.

Greptile's one remaining comment asks for a `selectAriaLabel` prop so
call-sites could override the label. We think the hardcoded label is
correct here and match existing practice: shared components whose
meaning is fixed own their label internally (`ThemeToggle.tsx`), while
components whose label depends on the data they render take it as a prop
(`CopyText.tsx`'s `ariaLabel`). This `<select>` always means "which page
section", at every call-site, so a prop no caller would set would be
unused API surface.

ui/src/components/PageTabBar.tsx

@@ -23,6 +23,7 @@ export function PageTabBar({ items, value, onValueChange, align = "center" }: Pa
         value={value}
         onChange={(e) => onValueChange(e.target.value)}
         className="h-9 rounded-md border border-border bg-background px-2 py-1 text-base focus:outline-none focus:ring-1 focus:ring-ring"
+        aria-label="Page section"
       >
         {items.map((item) => (
           <option key={item.value} value={item.value}>