Seat each hour in its own cell (#113)

The timeline's hour lines become a per-hour grid, in the month grid's visual
language: 24 rounded cells on surfaceContainer, separated by a 2dp gap that
reveals the surface behind them, so the hour boundary is negative space rather
than a stroke. The column's Card takes surface and the container colour moves
onto the cells.

- `HourLines.kt` becomes `HourGrid.kt`: `hourGridCells` draws the cells in the
  same `drawBehind` the lines used, under the blocks and above the column.
- The cell corner is the event chip's 4dp, not the month grid's 12dp, and is
  clamped to half the shorter side so a fit-the-day sliver or a narrow week
  column can't round into a lozenge.
- The hour gutter's label lift comes off its own line height instead of a
  hardcoded -6dp, so label and seam line up at any font scale.
- The preference keeps its stored key and its string keys; only the English
  source text and the Kotlin names change (`showHourLines` → `showHourGrid`).

Block placement is not left untouched, as the issue expected: a block gives up
the same half-gap at each end the cells do, or an on-the-hour event overhangs
the seam into its neighbours' cells. That inset follows through to the drag —
the floating copy takes the same gap off, so it stays the size of the block it
lifted off (#267), and the grab is measured off the placement rather than the
seated position. An end cut at midnight keeps none of it and stays against the
column edge.

Closes #113

CHANGELOG.md

@@ -20,6 +20,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   is not in that row. Event times across the month, week and day views are also
   set in a regular weight against the title's, so a time reads as a time rather
   than as part of the name next to it ([#219]).
+- **The hour lines in week and day view are now an hour grid.** The setting
+  that drew a faint separator line at each hour now seats every hour in its own
+  rounded cell, so an hour boundary reads as the seam between two surfaces
+  rather than a line drawn across one — the same negative space that separates
+  the month grid's days and the week's columns. An event that starts on the hour
+  fills its cell instead of overhanging the seam, and one that runs past
+  midnight still meets the edge of its column ([#113]).
 - The calendar titles now shorten instead of being cut off. When the full month
   name doesn't fit the top bar, the month and week views fall back to its
   three-letter form and the day view drops the weekday, rather than trailing off
@@ -1603,3 +1610,4 @@ automatically, with zero telemetry and no internet permission.
 [#248]: https://codeberg.org/jlmakiola/calendula/issues/248
 [#253]: https://codeberg.org/jlmakiola/calendula/issues/253
 [#273]: https://codeberg.org/jlmakiola/calendula/issues/273
+[#113]: https://codeberg.org/jlmakiola/calendula/issues/113