Accessibility: keyboard-reachable list rows, translated aria-labels, chart alt text

[iamgroot400]

Stacked on #43 (uses the starter-plan picker it adds), so this diff includes 834744f until that one merges — only 2c768d8 is new here.

Addresses the "Accessibility passes on the workout and chart screens" item in CONTRIBUTING.md's good-first-issues list. Three related gaps, all in how the app talks to assistive tech:

Unreachable list rows

Every clickable list row (<div className="item" onClick=...>, 16 of them across sheets.jsx, Plan/Admin/Library/Workout) was unreachable by keyboard and invisible to screen readers, a div has no role or tabIndex on its own. Swapped to a real <button> everywhere nothing else was nested inside it; the one exception (Library's exercise row, which holds its own "Plan" button) got role="button" + tabIndex + Enter/Space instead, since a button can't contain another button.

Untranslated aria-labels

Icon-only buttons across the app (steppers, calendar/week nav, delete/clear) had aria-label="Decrease" etc as raw English literals, every other user-facing string goes through t(), these didn't, so a screen reader in any of the other 11 languages announced English button names regardless of the profile's language. Wrapped them in t(). Left Admin.jsx's own three untranslated on purpose, that file has no i18n at all (admin tool, not the translated user-facing app), so partially translating it would be inconsistent with its own existing convention.

Charts with no accessible name

LineChart's <svg> had no accessible name at all, a screen reader got nothing from any chart in the app (body weight, 1RM, effort, everything under Stats). Added role="img" with a one-line aria-label (entry count, first value, last value). Heatmap's day cells went from title-only (not reliably read by screen readers) to real buttons with the same summary as an aria-label; empty cells are marked aria-hidden since they carry no information.

Check (the per-set done toggle) gained an optional label prop; the workout screen's set checkbox now says "Set 1 done" instead of just "checkbox, not checked".

Test plan

• npm test (192 passing)
• npm run build
• Clicked through in a dev browser: loaded a starter plan, confirmed every .item row is a real button (or div[role=button] for the one with a nested button) with a meaningful accessible name from its own text; logged a body weight entry and confirmed the chart's aria-label reads "70 kg, one entry"; confirmed the Library "Plan" button's stopPropagation still works standalone inside the new div[role=button] row