Let a dayTintProvider send its own colour

Four tones cannot keep a twenty-stop trip's legs apart, which is most of
the point of colour-coding one. A contribution can now carry `#rrggbb`
alongside `tone`, per region or as a shorthand.

The split of control is: the plugin picks the hue, the host picks the
weight. A colour rides the same per-theme, per-region alphas as a tone,
and its OKLCH lightness is clamped into a per-theme band first, so no
contribution can hand a user a day card they can't read. The band
brackets the built-in tones rather than tightening around them, so an
ordinary colour passes through untouched and a hex equal to a tone
renders exactly like that tone. Hue and chroma are never touched - it
bounds a colour, it doesn't boost one, so a near-white grey still lands
as a faint tint.

Only `#rrggbb` survives normalization, and that is a security boundary
rather than tidying: the value ends up inside a `color-mix()` in an
inline `background`, a shorthand that takes a layer list, so a string
closing the paren early could append a `url(...)` and beacon everyone
viewing the trip.

A region gets exactly one paint instruction - what it names beats the
shorthand, a colour beats a tone at the same level - so the client never
has a tie to break. A region naming a colour the host can't use degrades
to `default` like a bogus tone already did.

client/src/mobile/screens/trip/MTripShell.tsx

@@ -316,7 +316,7 @@ export default function MTripShell({
                   // The chip is mobile's day-number badge, so it follows badgeTone.
                   // Inactive chips only — an inline background would otherwise beat
                   // the active chip's bg-m-act class.
-                  style={active ? undefined : { background: dayTintBackground(tint?.badgeTone, '--day-tint-chip') }}
+                  style={active ? undefined : { background: dayTintBackground(tint, 'badge', '--day-tint-chip') }}
                   className={`flex-1 whitespace-nowrap rounded-full px-3 py-[5px] text-center text-[0.75rem] font-semibold ${
                     active ? 'bg-m-act text-m-actfg shadow-[0_6px_16px_-6px_rgba(0,0,0,.4)]' : 'text-m-ink'
                   }`}