feat(mobile): promote filter toggle when active + document 360px baseline

MOB-08: Search list pagination is already implemented (page_size 50,
"Load more schools" button + count) — no change needed; ticket closed.

MOB-10: Rather than build a full bottom-sheet filter modal (large
change, modal/focus-trap/scroll-lock infra), promote the existing
"Advanced" toggle to a coral pill labeled "Filters (n)" whenever
dropdown filters are applied. Users now see at a glance that the list
is being narrowed; the inline accordion remains the disclosure
mechanism. Adds aria-expanded for screen readers.

MOB-23: Add MOBILE.md at the repo root with the 360 px design baseline,
acceptance checks for any UI PR (no horizontal overflow, ≥44px tap
targets, no <11px visible text, iOS Chrome parity, safe-area-inset,
dvh), and the established component patterns. Playwright regression
test deferred — adding the dep for one test is heavier than the
current value warrants; documented as a future option.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

MOBILE.md

@@ -0,0 +1,71 @@
+# Mobile design baseline
+
+Mobile (≥55% of traffic) is the primary target for this app. Any new
+screen or component must be designed at the **360 px** viewport first
+and verified at three reference widths before merge.
+
+## Reference viewports
+
+| Width | Device class | Purpose |
+| --- | --- | --- |
+| 360 px | Low-end Android (Samsung A-series, older Pixels) | Hard floor — if it doesn't fit here it isn't shipping |
+| 390 px | iPhone 14 / 15 / 16 (38% of mobile traffic) | Primary iOS target |
+| 430 px | iPhone 16 Pro Max, large Android | Upper mobile bound |
+
+## Acceptance checks for any screen change
+
+Before raising a PR that touches user-visible UI, confirm at each
+reference width:
+
+1. **No horizontal overflow.** `document.documentElement.scrollWidth ===
+   window.innerWidth`. The most reliable check: in DevTools console run
+   ```js
+   document.documentElement.scrollWidth - innerWidth
+   ```
+   It must read `0`. Any positive number means something is bleeding
+   past the right edge — usually a fixed-width element, an inline-block
+   that didn't wrap, or a flex row missing `flex-wrap: wrap`.
+2. **Tap targets ≥ 44 × 44 px** on every interactive element (iOS Human
+   Interface Guidelines minimum). Probe with:
+   ```js
+   Array.from(document.querySelectorAll('a, button, [role=button], input, select'))
+     .filter(el => el.offsetParent)
+     .map(el => ({ t: el.innerText?.trim().slice(0,30), r: el.getBoundingClientRect() }))
+     .filter(o => o.r.width < 44 || o.r.height < 44)
+   ```
+3. **No text below 11 px** in any visible-by-default block. Decorative
+   demo content (illustrations, mocked previews) should either scale up
+   or be hidden under the `640 px` breakpoint — see `MOB-04` for the
+   pattern used on the home page's "What you'll see" section.
+4. **iOS Chrome bottom-bar parity.** The fixed `Navigation` bottom tab
+   bar already compensates for the auto-hiding URL bar via the Visual
+   Viewport API (`Navigation.tsx`). New fixed-bottom elements must
+   either use the same offset (read `var(--mobile-bar-offset)`) or sit
+   inside the existing tab-bar container.
+5. **Safe-area insets** on any new sticky/fixed chrome:
+   `padding-bottom: env(safe-area-inset-bottom)` for bottom-pinned UI,
+   `padding-inline: env(safe-area-inset-left/right)` for header-class
+   chrome that runs full bleed.
+6. **`dvh`, not `vh`.** iOS Safari's collapsing toolbar makes raw `vh`
+   units jump. Prefer `100dvh` (with a `100vh` fallback if you support
+   older engines) for any height that needs to track the visible
+   viewport.
+
+## Component patterns
+
+- **Hide-on-mobile decoration:** wrap with `@media (max-width: 640px) {
+  .x { display: none; } }` — examples in `HomeView.module.css`
+  (`.hiwVisual`), `MetricTooltip.module.css` (`.wrapper`).
+- **Right-edge scroll-fade for horizontal scrollers:**
+  `mask-image: linear-gradient(to right, #000 calc(100% - 28px), transparent);`
+  Drop the fade when scrolled to the end with a JS-toggled class — see
+  `SchoolDetailView.tsx`'s `sectionNavAtEnd` state for the pattern.
+
+## Automation (future)
+
+A Playwright regression test that asserts `docW === vw` at the three
+reference widths on `/`, `/rankings`, `/admissions`, `/compare`, and a
+representative `/school/:urn` page would catch overflow regressions
+immediately. Not added yet — Playwright isn't currently in the project
+dependency set, and the existing Jest setup doesn't compute layout.
+Worth adding if mobile overflow regressions recur.