feat(mobile): replace the phone Shell button with Enter, move Shell into Run

On phones the toolbar slot held "Shell", which starts a rarely-needed session
type. Sending Enter is a constant need on a touch keyboard, so the slot now
holds a dark blue Enter button and shell launching moves into the expandable
Run dropdown (Terminal / Shell, label "Run SH"). Desktop and tablet are
unchanged: the green Run Shell button stays exactly where it was.

Enter goes through xterm's own input path:

  coreService.triggerDataEvent('\r', true)

NOT through sendInput() or a direct POST to /input. localEchoEnabled defaults
to MobileDetection.isTouchDevice(), so on a phone the characters you type are
buffered client-side in the LocalEchoOverlay and have never reached the PTY.
The onData Enter branch in terminal-ui.js is what flushes that buffer before
sending \r. A bare \r submits an empty line and leaves the typed text stranded
on screen, which presents as "the Enter button does nothing". Replaying the
keypress reuses the overlay flush, the flushed-offset cleanup and the 80ms
text-before-CR ordering instead of reimplementing them.

Verified with local echo forced on: before the fix the overlay still held
"echo OLD_WAY" after Enter; after it, pendingText is empty and the command
executes in the pane.

The !important on the Enter button's colors is required, not habit: styles.css
nests its skin overrides inside `html:not([data-skin="og"]) { … }`, so a plain
.btn-toolbar there resolves to (0,2,1) and outranks .btn-toolbar.btn-enter at
(0,2,0). Without it the button renders in generic toolbar grey.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/mobile.css

@@ -875,19 +875,44 @@ html.mobile-init .file-browser-panel {
     margin-right: 0;
   }
 
-  /* Secondary action - Run Shell - right side */
+  /* Shell is NOT a toolbar button on phones — it moved into the Run dropdown
+     (Terminal / Shell), freeing this slot for Enter. Starting a shell is a rare,
+     deliberate act; sending Enter is a constant one, so the scarce phone real
+     estate goes to Enter. */
   .btn-toolbar.btn-shell {
-    flex: 0 0 auto;
-    background: transparent;
-    border: 1px solid rgba(255, 255, 255, 0.2);
-    color: #9ca3af;
-    order: 4; /* Right position */
+    display: none !important;
   }
 
-  .btn-toolbar.btn-shell:hover,
-  .btn-toolbar.btn-shell:active {
-    background: rgba(255, 255, 255, 0.1);
-    color: #fff;
+  /* Secondary action - Enter - right side. Takes the slot (and the order) the
+     Shell button used to hold, so the toolbar rhythm is unchanged. */
+  .btn-toolbar.btn-enter {
+    display: flex !important;
+    flex: 0 0 auto;
+    align-items: center;
+    justify-content: center;
+    min-width: 54px;
+    width: 54px;
+    white-space: nowrap;
+    padding: 0 8px !important;
+    overflow: hidden;
+    font-size: 0.65rem;
+    font-weight: 600;
+    letter-spacing: 0.01em;
+    /* !important is REQUIRED here, not defensive habit: styles.css nests its skin
+       overrides inside `html:not([data-skin="og"]) { … }`, so a plain `.btn-toolbar`
+       in that block resolves to (0,2,1) and outranks this (0,2,0) rule. Without
+       !important the button silently renders in generic toolbar grey. */
+    background: rgba(30, 58, 95, 0.85) !important;
+    border: 1px solid rgba(59, 130, 246, 0.45) !important;
+    color: #dbeafe !important;
+    order: 4; /* Right position — same slot Shell used to occupy */
+  }
+
+  .btn-toolbar.btn-enter:hover,
+  .btn-toolbar.btn-enter:active {
+    background: rgba(37, 74, 122, 0.95) !important;
+    border-color: rgba(59, 130, 246, 0.7) !important;
+    color: #fff !important;
   }
 
   /* Hide case selector on mobile - simplified toolbar */
@@ -895,27 +920,12 @@ html.mobile-init .file-browser-panel {
     display: none !important;
   }
 
-  /* Simplified toolbar layout — Run, Shell, and Case */
+  /* Simplified toolbar layout — Run, Enter, and Case */
   .toolbar-left .toolbar-group:first-child {
     width: 100%;
     gap: 8px;
   }
 
-  .btn-toolbar.btn-shell {
-    flex: 0 0 auto;
-    min-width: 54px;
-    width: 54px;
-    white-space: nowrap;
-    padding: 0 8px !important;
-    overflow: hidden;
-    font-size: 0 !important;
-  }
-
-  .btn-toolbar.btn-shell::after {
-    content: "Shell";
-    font-size: 0.65rem;
-  }
-
   /* Mobile case button - visible on mobile */
   .btn-toolbar.btn-case-mobile {
     display: flex !important;