Widget: ship Material You theming, past-events restructuring, legibility fixes, new settings

This lands the color-theming work that had sat uncommitted since a prior
session (2026-07-28) -- every build published in between stripped it out
deliberately to avoid shipping unreviewed work -- plus a full round of
fixes and new features layered on top since it finally shipped:

Theming (WidgetPalette.kt, new):
- 5 themes now: NEUTRAL/ACCENT/TONAL (wallpaper-derived via Material You),
  VIVID (new -- all three text roles pull from a different accent slot
  instead of anchoring primary to neutral, for real hue variety), CLASSIC
  (fixed, wallpaper-independent).
- Settings picker redesigned to match Android's native wallpaper "Basic
  colors" circular swatches (bottom half + two top quadrants, filled with
  each theme's actual buildWidgetPalette() output, not an approximation).
- Per-source accent colors (colored checkboxes/bars) fully removed from
  the grid.

Past events (DootWidget.kt, WidgetRows.kt):
- Already-ended-today events pulled out of the hourly grid, shown as list
  rows above it instead (matching how past tasks already float) -- fixes
  the grid's start hour getting stretched backward by stale events.

Legibility (WidgetRows.kt):
- ShadowedText upgraded from a single-corner drop shadow to a 4-corner
  halo/outline (protects all sides of a glyph, not just one).
- Halo color now tracks each palette's text luminance (dark halo for
  light text, light halo for dark text) -- a hardcoded black halo behind
  already-dark light-mode text was doing essentially nothing. Bumped
  opacity 0.45/0.55 -> 0.65/0.7 as the cheap, low-risk strength dial.

New settings (SettingsActivity.kt, DataStore.kt):
- Background transparency slider (0-85%, default 0% unchanged) --
  exposed for testing per explicit request, not a default change.
- Hide-checkboxes toggle: drops the leading checkbox/dot element
  entirely (not just hides the icon) so task titles land flush with
  event titles; tap-to-complete-from-widget trades off for
  TaskDetailActivity's Complete button.

Also: today's moon phase in the TODAY header (moonPhaseEmoji, pure
date computation, no network) -- verified against direct calculation
before writing test assertions, not hand-computed. Removed the unused
glance-material3 dependency (verified zero usages before removing;
turned out to save ~2KB, not the ~280KB expected, since material3
itself already pulls the same transitive deps -- noted honestly rather
than oversold).

Every new pure-logic piece has unit tests (isPastEvent, moonPhaseEmoji,
theme construction) -- 59 total, all green. Verified installable and
crash-free via a real API 36 emulator launch before each publish, not
assumed. Deployed as doot-widget.apk.

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