test(mobile): follow the 600px phone cut on the Duo branch

Rebased over #390, which moved the phone tier's cutoff from 430px to
600px. The palette's compound fold rule now lives in the 600-768px band
mobile.css pads, the cascade samples the palette inside that band, and
the closed iPhone Duo (466pt) is a phone rather than a small tablet while
the open one (626pt) stays a tablet. Comments in both stylesheets, the
device registry, CLAUDE.md and architecture-invariants say 600.

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

src/web/public/mobile.css

@@ -3853,7 +3853,7 @@ html[data-session-list="sidebar"] .session-sidebar .session-tab .tab-close {
 
 /* 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
+   phone block (under 600px) 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. */

src/web/public/styles.css

@@ -18128,14 +18128,14 @@ html[data-session-list="sidebar"][data-sidebar="collapsed"] .btn-sidebar-toggle
 }
 
 /* The palette is a .modal, so the generic rule above covers it everywhere
-   EXCEPT the 430-768px band, where mobile.css (loaded after this file) pads
+   EXCEPT the 600-768px band, where mobile.css (loaded after this file) pads
    it with the SHORTHAND `10vh 0.75rem 0`: that shorthand beats the generic
    rule on both sides, so this compound rule (0,2,0) restates that band's own
    gutters, 0.75rem at the side and none at the bottom, plus the fold strips.
    Scoped to the same band on purpose: unscoped, it ADDED 0.75rem on every
    other width, where the palette has no side gutter to compose with, and
    pushed the shell 6px off centre (measured at 393, 900 and 1400). */
-@media (max-width: 768px) and (min-width: 430px) {
+@media (max-width: 768px) and (min-width: 600px) {
   .modal.command-palette-modal {
     padding-right: calc(0.75rem + var(--fold-inline-end));
     padding-bottom: var(--fold-block-end);