fix(schedule): give the HTML export a constant week pitch

[atman-33]

What

The schedule's HTML export laid a week row out as three stacked blocks (day
numbers, bar lanes, milestone row) with no height of its own, so every week
was as tall as whatever it happened to hold. A week with a bar towered over an
empty one and the calendar lost its rhythm — the complaint behind T-0099.

The on-screen grid has given each day cell a minimum height all along; only
the export was missing it.

How

• A week row is now a single seven-column table: one td per day, holding the
  date line, a spacer sized to the week's lane count, and that day's
  milestones. Bars and arrows span columns, so they stay an absolutely
  positioned overlay across the whole week.
• Day cells get a 96px minimum height (a table cell's height is a minimum, so
  a week with more lanes than fit still grows), vertical column rules, and
  non-working shading over the whole cell rather than behind the date alone.
• The date and its non-working label share one fixed-height line, so bars start
  at the same offset in every cell.
• The weekday header's outer cell drops its padding — otherwise its column
  rules sit a few px off from the ones in the rows below.

Element geometry is untouched: it is still exactly buildLayout's output, as
§8.2 of the design note requires.

Trade-off

Taller rows fit fewer weeks per printed page — roughly 12 before, 6–7 now on
A4 landscape. Accepted for readability; recorded as §16.12 in the vault design
note.

Verification

• Rendered the vault's test schedule and measured the output: all week rows
  97px, header and body column rules aligned to the pixel, bars at the head
  offset and milestones one lane below with no overlap.
• npx vitest run (124 passed) and npm run build green. One existing test
  asserted the old daynum ... today class arrangement and was updated to the
  new daycell ... today.

🤖 Generated with Claude Code

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✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch feature/schedule-export-cell-height

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