a11y: accessible labels and validation ARIA on form inputs

[ChristopherRotnes]

Summary

Adds programmatic labels and validation state to the custom inputs used throughout the app so screen reader users know what each field is for and hear errors without manually hunting for them.

• ExerciseRow — aria-label="Øvelsenavn" on the exercise name input; aria-label, aria-invalid, and aria-describedby on sets/reps inputs; an off-screen error <span> announces "Ugyldig antall – skriv inn 1 til 99" when a value is invalid
• MusclePicker — the help text span gets a stable id; the picker wrapper gets aria-describedby pointing to it so screen readers read the instructions on focus; aria-hidden="true" added to the decorative body outline <g> and FRONT/BACK <text> (same pattern as bodymap.jsx in PR a11y: set HTML lang, add reduced-motion CSS, hide decorative SVG elements #84)

Test plan

• Tab into an exercise row name field — screen reader announces "Øvelsenavn"
• Tab into a sets or reps field — screen reader announces e.g. "Sett for Knebøy"
• Enter an invalid value (e.g. 0 or 999) and submit — screen reader announces the error without manual navigation
• Tab into the MusclePicker — screen reader reads the "Klikk muskel: av → primær → sekundær → av" instructions

Closes #70, #77, #83

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