fix(web): align week header and grid columns

[Uarmagan]

Summary

• Align the week day headers, all-day row, and timed grid by putting them on the same seven-column layout track.
• Keep the week grid usable at compact desktop widths by moving the week body into a horizontal scroll area.
• Hide the native white horizontal scrollbar and replace it with a small Compass-styled scroll rail that only appears when horizontal scrolling is available.
• Remove the extra timed-grid scrollbar gutter while keeping the inner vertical grid scrollbar visually hidden.
• Keep click-and-drag timed drafts on the day column under the pointer by using measured grid coordinates consistently.
• Replace the temporary alignment-probing tests with focused browser coverage for header/grid alignment, hidden scrollbar behavior, horizontal scrolling, and sidebar-open timed draft placement.

Why

The week header, all-day band, and timed grid could drift or appear visually broken when the calendar was narrow, especially with the sidebar open. The layout mixed different distribution methods and also reserved scrollbar space inside the timed grid, so the visible grid could look wider than the actual day columns.

After the columns were unified, compact desktop widths still needed a better overflow story. The browser's native bottom scrollbar was functional but visually loud and did not match the dark Compass UI. This PR keeps horizontal scrolling available, hides the native bar, and adds a restrained in-app rail to signal that more days are available off-screen.

A separate drag regression came from mixing old sidebar-width X-coordinate correction with the newer measured grid coordinates. Pointer X values were shifted before date calculation, so a drag on one day could create the draft on another. The date-mapping path now uses the actual browser pointer coordinate against measured grid bounds.

Reviewer Notes

• Columns now uses CSS Grid with seven equal day tracks instead of flex distribution.
• DayLabels, StyledAllDayColumns, and StyledGridCols all build on the shared column primitive.
• WeekGridScrollArea owns the horizontal overflow affordance: it hides the native horizontal scrollbar, observes scrollability, and renders the small bottom rail/thumb only when the week track overflows.
• The horizontal edge gradients were removed; the rail is the only scroll affordance.
• Time labels are not pinned during horizontal scrolling in this PR. They move with the grid content; making them sticky is intentionally left for later.
• #mainGrid keeps --scrollbar-width: 0px and the scrollbar utility so the actual rendered vertical scrollbar width is zero instead of falling back to Chrome's native white scrollbar.
• Draft creation, drag, resize, and sidebar drop date calculations now pass raw pointer X into the measured grid calculation instead of subtracting the sidebar width first.

Validation

• Viewed http://localhost:9080/week locally and confirmed the native bottom scrollbar is hidden and the subtle rail appears at compact width.
• Checked the rendered page directly with Playwright: compact width reports horizontal scrollbar height 0px, scroll rail visible, and horizontal scrolling works; wide width hides the rail because no horizontal overflow is present.
• Confirmed the inner timed grid scrollbar renders at width 0px after restoring the scrollbar utility on #mainGrid.
• Confirmed the time labels are no longer counter-positioned during horizontal scrolling.
• bunx playwright test e2e/navigation/week-view-layout.spec.ts e2e/timed/create-event-mouse.spec.ts --project=chromium-desktop
• bun run type-check
• bunx biome check e2e/navigation/week-view-layout.spec.ts packages/web/src/common/constants/web.constants.ts packages/web/src/views/Calendar/Calendar.tsx packages/web/src/views/Calendar/styled.tsx packages/web/src/views/Calendar/components/Grid/WeekGridScrollArea.tsx packages/web/src/views/Calendar/components/Grid/Columns/TimesColumn/styled.ts
• git diff --check

Lint Note

• bun run lint is not currently clean in this checkout. It now exits non-zero on an unrelated backend duplicate test-hook error at packages/backend/src/auth/services/google/google.auth.service.test.ts:37, along with the existing warning backlog. That backend file is not changed by this PR.