docs: add homepage search design spec

docs/superpowers/specs/2026-04-11-homepage-search-design.md

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+# Homepage Search — Design Spec
+
+**Date:** 2026-04-11  
+**Status:** Approved
+
+## Overview
+
+Wire up the `SearchBar` on the homepage so it performs a live station search and renders a map + station list inline, below the search bar. No navigation away from the page. All existing components (`LeafletMap`, `StationList`, `useStations`) are reused as-is, with minimal targeted changes.
+
+---
+
+## Component Changes
+
+### `SearchBar.vue`
+
+- Add a **fuel type** `<select>` with options: E10, E5, Diesel (B7), Premium Diesel (B7), B10, HVO
+- Add a **radius** `<select>` with options: 2, 5, 10, 20 miles (default: 10)
+- Change the `search` emit payload from a bare postcode string to `{ postcode, fuelType, radius }`
+- **Remove** the debounced `onInput` handler — search fires on button click only (debounce on a multi-param form causes confusing mid-type fetches)
+- Button stays disabled when postcode is empty
+
+### `LeafletMap.vue`
+
+- Add a `defaultOpen` Boolean prop (default `false`) for backwards compatibility
+- When `defaultOpen` is `true`, initialise the map on `onMounted` instead of waiting for the toggle
+- Toggle button remains so users can collapse the map
+- Auto-renders markers when `stations` prop is populated after mount
+
+### `Home.vue`
+
+- Import `useStations` and hold `sort` ref (default `'price'`)
+- On `@search` from `SearchBar`, call `useStations.search({ postcode, fuelType, radius, sort })`
+- Add a results `<section>` directly below the hero section — full width, `max-w-7xl` container, `px-6 py-10` padding
+- Results section only renders when a search has been attempted (`stations.length || loading || error`)
+- Results layout (top to bottom):
+  1. `LeafletMap` with `:stations="stations"` and `:default-open="true"` — `h-80` height
+  2. `StationList` with `:stations="stations"` `:current-sort="sort"` `@sort="onSort"`
+- **Loading state:** spinner centred in the results section, replaces map+list
+- **Error state:** inline error message (`error.general[0]` or fallback text), replaces map+list
+- `onSort` handler updates `sort` ref and re-runs `search()` with last used params (store params in a `lastParams` ref)
+
+---
+
+## Data Flow
+
+```
+SearchBar emits { postcode, fuelType, radius }
+  → Home.vue calls useStations.search({ ...params, sort })
+    → API GET /stations?postcode=...&fuel_type=...&radius=...&sort=...
+      → stations ref populated
+        → LeafletMap renders markers (auto-open)
+        → StationList renders cards
+```
+
+---
+
+## What Is Not Changing
+
+- `StationCard.vue` — no changes
+- `useStations.js` — no changes
+- API layer — no changes
+- Dashboard usage of `LeafletMap` and `StationList` — unaffected (defaultOpen defaults to false)
+
+---
+
+## Out of Scope
+
+- Saving/favouriting stations from homepage (dashboard feature)
+- Geolocation "use my location" button
+- Pagination of results