fix(#1487): BYOP modal — bounded header, no body occlusion (#1493)

## Fixes #1487

Reporter (@EldoonNemar): "The dialog text can't be seen due to the title
bar being massive."

### Root cause
`.byop-header` swelled to ~73px on mobile because:
1. `position: sticky` + `margin: -24px -24px 12px` assumed `.modal`
desktop padding (24px) — but `.modal` switches to 16px padding at
mobile, so the sibling-margin pushed the description paragraph UP into
the sticky-pinned header band, occluding it.
2. `.btn-icon` close button floors at 48×48 (touch target) → forced
header height ≥48px+padding.
3. H3 inherited a default emoji line-height that added more height on
platforms with tall emoji ascent metrics.

### Fix (`public/style.css`)
- Drop full-bleed negative-margin gymnastics — header uses normal
in-flow padding (`4px 0`); `.modal` padding handles inset.
- `max-height: 48px` on header so emoji ascent / btn-icon floor can't
blow it past safe range.
- Bound H3 explicitly (`font-size: 1rem; line-height: 1.3`).
- Override `.byop-x` to compact 32px visual size; preserve ≥44px
effective tap target via invisible `::before` pad (a11y safe).

### Verification
Hot-swapped onto staging, CDP-measured both viewports:

| viewport | hdrH | descTop ≥ hdrBottom | result |
|---|---|---|---|
| 390×844 mobile | 41px (was 73) | 341 ≥ 329 ✅ | clean |
| 1280×800 desktop | 41px | 318 ≥ 306 ✅ | clean |

### TDD
- **Red commit**: bb1a9f48 — `test-issue-1487-byop-modal-layout-e2e.js`
asserts header ≤56px AND description top ≥ header bottom on both
viewports. Pre-fix: header=73px ⇒ FAIL.
- **Green commit**: 72a69b3e — CSS fix; assertions all pass against
hot-swapped staging.
- E2E added: `test-issue-1487-byop-modal-layout-e2e.js`; wired into
`.github/workflows/deploy.yml` e2e job.

### Screenshots
Before (mobile): description "Paste raw hex bytes..." clipped by
oversized header. After: header 41px, description fully visible above
textarea.

---------

Co-authored-by: corescope-bot <bot@corescope.local>

.github/workflows/deploy.yml

@@ -341,6 +341,7 @@ jobs:
           CHROMIUM_REQUIRE=1 BASE_URL=http://localhost:13581 node test-channels-add-modal-e2e.js 2>&1 | tee -a e2e-output.txt
           CHROMIUM_REQUIRE=1 BASE_URL=http://localhost:13581 node test-channels-share-color-e2e.js 2>&1 | tee -a e2e-output.txt
           CHROMIUM_REQUIRE=1 BASE_URL=http://localhost:13581 node test-channels-ws-batch-e2e.js 2>&1 | tee -a e2e-output.txt
+          CHROMIUM_REQUIRE=1 BASE_URL=http://localhost:13581 node test-issue-1487-byop-modal-layout-e2e.js 2>&1 | tee -a e2e-output.txt
 
       - name: Collect frontend coverage (parallel)
         if: success() && github.event_name == 'push'