Refactor: replace prop drilling with composables for shared display state

[irees]

Background

The Census Demographics PR (#338) added several new pieces of URL-backed state and display preferences to app/pages/tne.vue (choroplethElement, shadeByDensity, onlyWithStops, unitSystem, etc.). Each one is plumbed as an individual v-model or prop:

• ~10 v-models on cal-filter
• ~15 props on cal-map
• Further plumbing inside cal-map → cal-legend and cal-map-viewer-ts

Adding one new knob (like onlyWithStops) currently means editing 4–5 files. The problem is structural: state that belongs together (choropleth display, display preferences) is being plumbed piece-by-piece instead of shared.

Plan: two composables

Vue's idiomatic answer here is composables that own state, not a giant state object passed as a prop. The URL is already the source of truth — composables that read/write route.query give every consumer the same view without prop drilling.

1. useScenarioUrlState()

Owns the URL-backed display state currently scattered across tne.vue:

• showAggAreas, aggregateLayer, choroplethElement, shadeByDensity, onlyWithStops
• Possibly: dataDisplayMode, hideUnmarked, flexServicesEnabled, flexColorBy

tne.vue, cal-filter, and cal-map each call the composable directly. Removes ~10 v-models on cal-filter and several props on cal-map.

2. useDisplayPreferences()

Cross-cutting bits that lots of components want:

• unitSystem (currently threaded tne → map → legend / map-viewer-ts / census-panel)
• isAllDayMode
• Any other display-only flags that descendants of cal-map need

Removes the chain plumbing through cal-map.

Out of scope

• No Pinia / app-wide store. URL is already canonical; composables are sufficient.
• No mega ScenarioState object passed as a prop. Breaks fine-grained reactivity, conflicts with v-model, hides what each component depends on.
• Scenario inputs (bbox, dates, geographyIds) and scenario filters (agencies, route types, weekdays) are already mostly bundled into scenarioConfig / scenarioFilter computeds. Leave them.
• Selection state (selectedAggregationGeoid, selectedPanelData) is localized to tne.vue ↔ cal-census-panel. Already small after recent collapse. Leave.
• Leaf-component props. For low-fan-out per-component data, explicit props are still nicer to read. Composables are for the high-fan-out shared state.

Acceptance criteria

• cal-filter no longer receives the choropleth/aggregation v-models from tne.vue; calls the composable directly.
• cal-legend, cal-map-viewer-ts, and cal-census-panel no longer receive unitSystem as a prop.
• Adding a new URL-backed display flag is a one-file change.
• No regression in URL state persistence or query-string format.
• Lint, typecheck, tests pass.

Suggested order

1. useScenarioUrlState first — biggest immediate impact.
2. useDisplayPreferences second — smaller win, more cross-cutting.
3. Stop. Re-evaluate after these land before reaching for more.

Why now

Punt until #338 merges. That PR introduced the bulk of the new plumbing and is already complex enough; doing both at once risks blurring the diff and making review harder.