feat(mobile): chat input keybinds + safe-area + tap targets + overflow safety

- ChatInput: e.nativeEvent.isComposing early-return added (CJK IME
  safety — first Enter of a composition no longer submits). Bare-Enter
  send path gated by !isMobile so mobile inserts a newline; send is
  button-only. Cmd/Ctrl+Enter and Shift+Cmd/Ctrl+Enter retained as
  desktop secondary bindings. Placeholder is now viewport-aware. Outer
  wrapper gets paddingBottom: env(safe-area-inset-bottom) so iOS home
  indicator doesn't overlap.
- MessageBubble: ActionRow buttons (Copy / Regenerate / Fork / Trash)
  bumped to max-md min-h/min-w 44px; opacity-100 on mobile so actions
  don't hide behind a hover-to-reveal pattern. User bubble and
  assistant content wrapper gain break-words + min-w-0 so long
  unbreakable strings (URLs / paths) wrap rather than blowing out
  the column on narrow viewports.
- ChatPane: queued-message dropdown + close X + Stop-generating button
  hit max-md 44px sizing.
- ChatTabBar: per-tab X, +/History/Close-pane action buttons hit
  max-md 44px. Tab close X is force-visible on mobile (no
  hover-to-reveal).
- M8: CodeBlock / Markdown tables / ToolCallCard already wrap
  overflow-x-auto pre-existing — no source change needed there; the
  break-words + min-w-0 additions above are the new defensive layer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/components/ChatTabBar.tsx

@@ -115,7 +115,7 @@ export function ChatTabBar({
                     e.stopPropagation();
                     onRemoveTab(chat.id);
                   }}
-                  className="p-0.5 hover:bg-muted rounded opacity-0 group-hover:opacity-60 hover:!opacity-100 shrink-0"
+                  className="inline-flex items-center justify-center p-0.5 hover:bg-muted rounded opacity-0 group-hover:opacity-60 hover:!opacity-100 shrink-0 max-md:min-h-[44px] max-md:min-w-[44px] max-md:opacity-100"
                   aria-label="Close tab"
                 >
                   <X size={10} />
@@ -161,7 +161,7 @@ export function ChatTabBar({
         <button
           type="button"
           onClick={onNewChat}
-          className="p-1 rounded text-muted-foreground hover:bg-muted hover:text-foreground"
+          className="inline-flex items-center justify-center p-1 rounded text-muted-foreground hover:bg-muted hover:text-foreground max-md:min-h-[44px] max-md:min-w-[44px]"
           aria-label="New chat"
           title="New chat"
         >
@@ -171,7 +171,7 @@ export function ChatTabBar({
           type="button"
           onClick={onShowHistory}
           className={cn(
-            'p-1 rounded text-muted-foreground hover:bg-muted hover:text-foreground',
+            'inline-flex items-center justify-center p-1 rounded text-muted-foreground hover:bg-muted hover:text-foreground max-md:min-h-[44px] max-md:min-w-[44px]',
             pane.kind === 'empty' && 'text-foreground bg-muted/50'
           )}
           aria-label="Session history"
@@ -183,7 +183,7 @@ export function ChatTabBar({
           <button
             type="button"
             onClick={onRemovePane}
-            className="p-1 rounded text-muted-foreground hover:bg-muted hover:text-foreground"
+            className="inline-flex items-center justify-center p-1 rounded text-muted-foreground hover:bg-muted hover:text-foreground max-md:min-h-[44px] max-md:min-w-[44px]"
             aria-label="Close pane"
             title="Close pane"
           >