feat(web): two-row route form, source always editable

[cafca]

Summary

Reshape the web route input to mirror the mobile RouteCard so the user can change source or destination at any time without first cancelling the route.

• Two always-tappable rows (origin, destination), each with inline edit and a shared results dropdown.
• Drop the `if (route.data) return;` guard in `setRoutePoint` — map clicks now write to whichever row is currently being edited; legacy origin-then-destination fill remains the fallback.
• Swap button on the right of the destination row, visible once destination is set.
• Gear becomes a "Preferences" text button in the footer alongside "Save start as home" / the home toggle + reset.
• New `setOrigin` / `setDestination` / `swapWaypoints` helpers; origin and destination marker-drag handlers now route through them so `editingField` stays consistent and the recompute logic is shared.
• `applyStartAtHome` and `handleSaveHome` clear `editingField` so the home name renders instead of an empty input on first load for users with Start at home enabled.
• `clearRoute` resets `editingField` (to `'origin'` when origin is null, `null` when origin auto-fills from home).

Depends on #76 for the dev-server proxy fix used while previewing against beebeebike.com — independent change, can land in either order.

Test plan

• `just test-web` — 76 / 76 (4 new tests around swap + editingField).
• `just build-web` — clean.
• Local preview against the dev stack: set origin via search, set destination via search, change either after the route computes without clicking ×, swap origin/destination, drag origin and destination markers, toggle Start at home on/off.