UI-01: persistent 4-tab shell with a shared background map

Replaces the push/pop stack rooted at Record with a StatefulShellRoute
(Map/Rides/Plan/Settings), each tab keeping its own navigator so Trip Detail
and the route planner push within their own branch. AppShell/ShellScaffold
host one shared RideMap instance behind every tab -- full opacity and
interactive on Map, dimmed and non-interactive elsewhere -- so the camera
position and live path survive a tab switch instead of being refetched.
Every tab's AppBar/header is removed per the redesign's no-chrome mandate.

Fixes a bug this surfaced: RideMap rendered a structurally different tree
for empty vs. non-empty points, which crashed once the map became a
long-lived shell background instead of a fresh per-screen widget. Unified
the background-map tree shape so it no longer remounts mid-recording.

Verified on Medium_Phone_API_35: all four tabs, live recording surviving
tab switches with the camera preserved, and the real HOME-key lifecycle
tile-drop from V3-04 still firing correctly in the shell context.

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

docs/ui-redesign/README.md

@@ -19,7 +19,7 @@ v3 held to.
 
 | # | Ticket | Size | Depends on | Status |
 |---|---|---|---|---|
-| [UI-01](UI-01-tab-shell-background-map.md) | Persistent tab shell with an always-visible background map | L | — | Not started |
+| [UI-01](UI-01-tab-shell-background-map.md) | Persistent tab shell with an always-visible background map | L | — | Done |
 | [UI-02](UI-02-offline-skeleton-map.md) | Offline / no-connection skeleton map | S | UI-01 | Not started |
 | [UI-03](UI-03-glass-component-kit.md) | Shared floating-glass component kit | M | UI-08 | Not started |
 | [UI-04](UI-04-customizable-hud-widgets.md) | Customizable HUD telemetry widgets (drag, resize, visibility toggle) | L | UI-03 | Not started |