feat: support frontier defensive structures

[prayujt]

Outcome

• adds watchtower and fort rendering, construction controls, ownership handling, upgrades, and demolition for standalone neutral-tile structures
• requires an owned army on the selected tile and validates the configurable placement radius against owned settlement footprints
• adds explicit View tile and View army controls, plus map-marker toggling, so occupied frontier tiles remain easy to inspect and build on
• displays current and next-level watchtower vision and fort or settlement defense values from game config
• surfaces active battle defense bonuses and updates generated frontend contracts

Depends on city-io/backend#27.

Verification

• npm run check
• npm run build
• npx prettier --check src/routes/game/+page.svelte
• npx eslint src/routes/game/+page.svelte
• git diff --check

The repository-wide npm run lint still reports pre-existing Prettier issues outside these changed source files.

[prayujt]

Fixed in 3a83c80: standalone building tiles no longer render the empty-tile fallback or a competing forces column. The compact building inspector keeps its full width, while armies on the tile remain accessible through View army. Verified with svelte-check, ESLint, and a production build.