fix(mobile): fold padding keeps phone sheets flush, scopes the palette rule, caps the response viewer under 430px

Three cascade problems in the fold reserved-region CSS, each measured by
computed style in headless Chromium (styles.css + mobile.css in index.html
link order):

- The unconditional .path-picker-overlay / .path-preview-overlay fold rules at
  the end of the file beat the `padding: 0` both overlays set under 600px, so
  every phone got a 16px and 18px gutter on dialogs built flush (393 and 500px:
  edges floating off the screen). The fold strip is now restated on a ZERO
  base inside the same media query: 0/0 without a fold, the strip alone with
  one, 16/18 plus the strip from 626px up as before.
- .modal.command-palette-modal was unscoped, so outside the 430-768px band
  (where mobile.css pads the palette with a shorthand) it ADDED 0.75rem with
  no gutter to compose with and pushed the shell 6px off centre at 393, 900
  and 1400px, while inside the band the shorthand beat the generic .modal rule
  on the bottom side and the palette lost its block-end gutter. The compound
  rule now lives inside that band and restates both sides.
- The tabletop cap on .response-viewer lost to mobile.css's `max-height:
  92dvh` under 430px (same specificity, later file). mobile.css now carries an
  identical twin at its end.

test/foldable-layout.test.ts simulates the padding cascade across both files
at every breakpoint, with and without the fold rules, and requires the two to
differ by exactly the fold strip; it also pins the palette rule to the band
mobile.css keys on and the response-viewer twin to the styles.css value. Its
model reproduces the Chromium numbers, and against the pre-fix stylesheets it
fails on all three problems.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/web/public/mobile.css

@@ -3850,3 +3850,15 @@ html[data-session-list="sidebar"] .session-sidebar .session-tab .tab-close {
     transition: none;
   }
 }
+
+/* Folding devices, tabletop pose: cap the response viewer to the bottom
+   segment. Twin of the rule at the end of styles.css, needed here because the
+   430px block above sets `max-height: 92dvh` on the same selector at the same
+   specificity, and this file loads later, so the styles.css copy loses on a
+   phone-width foldable. Keep the value identical; test/foldable-layout.test.ts
+   compares them. */
+@media (vertical-viewport-segments: 2) {
+  .response-viewer {
+    max-height: min(88vh, env(viewport-segment-height 0 1, 88vh));
+  }
+}