feat(insights): add a weekday by hour focus heatmap to the overview

[kYem]

Fixes ENG-141

Adds a weekday × hour focus heatmap to Insights → Overview with a one-line takeaway ("Most focused: Tuesday mornings"). The hour and weekday bar charts stay in Advanced Analytics.

Decisions:

• Counts sessions, not minutes. Completed work sessions, bucketed by the local time they ended (same as the existing charts); sessions are nearly all one length.
• Parts of day: morning 5–12, afternoon 12–5, evening 5–9, night 9 PM–5 AM. Hours after midnight count toward the night before, so a Tuesday 10 PM–2 AM habit reads "Tuesday nights".
• Needs 10 sessions before naming a peak; below that it says "Complete a few more sessions to see your pattern".
• Colour scale is the theme's primary at rising strength (color-mix), shared with the Advanced Analytics bars. The old 200–600 shades were near-identical greys in glass. Empty cells are outlines, not the lightest shade.
• Cells are buttons with roving tabindex (arrow keys, Home/End) and a label like "Tue 9 AM–10 AM · 4 sessions"; Biome rejects role="grid" on a table.

Screenshots (all 4 themes, light and dark) to be attached.