Accessibility and dark mode pass

[os-warren]

Post-v1 — v1 is M1+M2.

Scope

• Keyboard path through the whole primary flow: reach My Week, move down the list, complete, undo — without a mouse. This is the daily interaction; it should be the best keyboard experience in the product.
• Screen-reader labels on every status badge, source badge and action. A status conveyed only by colour is not conveyed.
• Contrast audit across light and dark on every chart fill, badge and label. Light fills with white text are the usual failure — they read fine on the author's screen and nowhere else.
• Focus states that survive the dark palette.
• Status colours must remain distinguishable in greyscale and to the common colour-vision deficiencies: open / in-progress / done / skipped / cancelled cannot rely on hue alone.

Constraints

• Dark mode is a token swap, not a second stylesheet. Define the light palette on bare :root and redefine only the tokens under the dark selector.
• Do not introduce a colour whose only definition lives inside a media query.