[W] lighten map text in dark mode

[Kelrap]

In dark mode, the text doesn't stand out well against the background. Make it a lighter gray.

TO TEST

After implementation, include a list of changes to be looked over

• Enable dark mode
• View map

[wcjord]

Taking this one.

[wcjord]

Implementing the vector-tile approach (OpenFreeMap). A single vector source styled client-side puts the dark map's label contrast and water color under our control with no second request, which is what we wanted here. Researching the exact vector_map_tiles + OpenFreeMap wiring, the flutter_map 8.x version compatibility, and the Flutter-web (CanvasKit) rendering cost first, then implementing.

[wcjord]

Punting on this for now. The vector-via-OpenFreeMap route is implemented and compiles for web, but the initial style/tile resolve is slow and the CanvasKit-web rendering cost is unproven and risky under throttling, so it is not the quick win we wanted. Parked the working spike as draft PR #7150 to pick up later (self-host tiles, tune the vector layer, or fall back to a Stadia raster stopgap). Leaving this issue open.

[wcjord]

Deferring this until the current [W] backlog is cleared.

The dark-mode map labels are baked into the Phase 1 raster tiles (CartoDB Dark Matter), so lightening just the text would need a second labels layer that doubles dark-mode tile requests. Per the tile strategy (world-map-tiles.instructions.md), bright, legible labels are the intended payoff of the Phase 2 vector migration, where label color and weight become client-side style properties at no added tile cost. Rather than ship a stopgap that works against that plan, we will fold this into Phase 2 (or revisit with a cheap interim tweak) once the higher-priority [W] items are done.