MiniBarChart empty state: add aria-label and localize the placeholder text

[tsouth89]

Context

The empty state in apps/desktop-tauri/src/components/MiniBarChart.tsx:28 renders a bare <span>No data</span>. Two small issues:

1. A11y — the chart container has no role="img" / aria-label, so the empty state is not described to assistive tech.
2. i18n — "No data" is hardcoded English while the rest of the app localizes via useLocale() / t(...).

Task

• Give the empty-state container a role="img" and an aria-label describing what is empty (include label when present).
• Replace the hardcoded "No data" with a localized string via the existing locale system.

Acceptance criteria

• Adds a new locale key (follow the existing key pattern; this repo is English-only, so only en-US.ftl plus the key registry need updating — see pointers).
• The empty state exposes an accessible name.
• No visual regression for the populated chart path.

How to test

pnpm --dir apps/desktop-tauri test
pnpm --dir apps/desktop-tauri run build

Pointers

• Component: MiniBarChart.tsx:24
• Locale strings live in rust/src/locale/en-US.ftl and the frontend key registry under apps/desktop-tauri/src/i18n/. Grep an existing key (one already used with t(...)) to see every place a new key must be added.