UI-08: migrate theme to Modern Professional Dark

Replaces V3-16's safety-orange identity with the palette actually backing
every fetched Stitch mockup (confirmed by hex-matching against all three
design systems in the Stitch project, not by trusting its declared default).
Ground drops to #0a0a0c with a three-level surface/card stack, primary
becomes Professional Blue (#4090fe), and reference readings move to a new
tertiary orange since blue is no longer free for that role. Re-verified
theme_test.dart's AA contrast assertions against the new values (all pass
with wide margin) and left mounted mode's separate daylight palette alone,
per the ticket's own reasoning.

Also resolves the ticket's named risk: RideMap's speed-gradient hardcoded
hex literal, which would have washed to a single hue now that primary is
blue, now lerps from the theme's own secondary token instead.

Inter is not wired in -- no bundled font asset exists to back the family
name, and setting one without an asset would silently fall back to Roboto.
Documented as a deliberate follow-up rather than a silent gap.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Xki7YAcc2TiN2PRZJ2tXr

docs/ui-redesign/README.md

@@ -26,7 +26,7 @@ v3 held to.
 | [UI-05](UI-05-map-hud-record-screen.md) | Map HUD: Record screen redesign | M | UI-01, UI-03, UI-04 | Not started |
 | [UI-06](UI-06-plan-and-route-planning.md) | Plan & Route Planning redesign | M | UI-01, UI-03 | Not started |
 | [UI-07](UI-07-rides-history.md) | Rides History redesign | M | UI-01, UI-03 | Not started |
-| [UI-08](UI-08-theme-modern-professional-dark.md) | Theme migration to Modern Professional Dark | S | — | Not started |
+| [UI-08](UI-08-theme-modern-professional-dark.md) | Theme migration to Modern Professional Dark | S | — | Done |
 | [UI-09](UI-09-monochrome-dark-map-tiles.md) | Monochrome dark map tiles | S | — | Not started |
 
 ## Dependencies