fix(iPad): CJK input positioning, paste dialog, and voice dictation duplication

Three iPad-specific issues fixed:

1. CJK input hidden behind keyboard: updateLayoutForKeyboard() gate changed
   from screen-size to touch-device detection. On iPad, CJK textarea (always
   position:fixed) gets bottom offset computed from keyboard HEIGHT directly
   instead of keyboardOffset (which depends on visualViewport.offsetTop that
   iOS adjusts when the CJK textarea receives focus). Toolbar/accessory bar
   transforms remain phone-only (they're normal-flow on iPad).

2. Paste dialog invisible on iPad: paste overlay CSS was inside
   @media (max-width: 430px) phone breakpoint — iPad (≥768px) had no styling.
   Extracted to universal section alongside keyboard accessory bar styles.

3. Voice dictation character duplication (Doubao/third-party IME):
   iOS voice dictation does NOT fire composition events (WebKit Bug 261764).
   Text arrives as bare input events; refinement is a delete→reinsert cycle.
   Rewrote CJK input handler with two-tier debounce:
   - Keyboard typing (no delete/replacement events): 150ms debounce
   - Dictation mode (deleteContentBackward or insertReplacementText detected):
     1500ms debounce, persists 3s to cover multi-word dictation
   - Composition path (compositionend): immediate flush, unchanged
   - Keydown singles/Enter/Esc/Ctrl: immediate, unchanged
   Also: keep cjkActive=true on blur while CJK is visible (prevents xterm
   from processing duplicate input when iOS dictation UI steals focus);
   keydown single-char sends tracked via timestamp to suppress the echo
   input event that third-party IMEs fire despite preventDefault.

src/web/public/mobile.css

@@ -1065,85 +1065,7 @@ html.mobile-init .file-browser-panel {
   }
 
   /* Paste overlay for iOS clipboard access */
-  .paste-overlay {
-    position: fixed;
-    inset: 0;
-    background: rgba(0, 0, 0, 0.6);
-    z-index: 10000;
-    display: flex;
-    align-items: flex-start;
-    justify-content: center;
-    padding-top: 15vh;
-  }
-
-  .paste-dialog {
-    background: var(--bg-secondary, #1e1e2e);
-    border: 1px solid var(--border-color, #444);
-    border-radius: 12px;
-    padding: 12px;
-    width: calc(100% - 24px);
-    max-width: 400px;
-  }
-
-  .paste-textarea {
-    width: 100%;
-    min-height: 80px;
-    max-height: 200px;
-    background: var(--bg-primary, #0d0d14);
-    color: var(--text-primary, #e0e0e0);
-    border: 1px solid var(--border-color, #444);
-    border-radius: 8px;
-    padding: 8px;
-    font-family: inherit;
-    font-size: 16px;
-    resize: none;
-    box-sizing: border-box;
-  }
-
-  .paste-textarea:focus {
-    outline: none;
-    border-color: var(--accent-color, #7aa2f7);
-  }
-
-  .paste-actions {
-    display: flex;
-    justify-content: flex-end;
-    gap: 8px;
-    margin-top: 10px;
-  }
-
-  .paste-cancel, .paste-new, .paste-send, .paste-image {
-    padding: 8px 18px;
-    border: none;
-    border-radius: 8px;
-    font-size: 14px;
-    cursor: pointer;
-  }
-
-  /* Image attach button — left-aligned, accent outline */
-  .paste-image {
-    margin-right: auto;
-    background: var(--bg-tertiary, #333);
-    color: var(--accent-color, #7aa2f7);
-    border: 1px solid var(--accent-color, #7aa2f7);
-  }
-
-  .paste-cancel {
-    background: var(--bg-tertiary, #333);
-    color: var(--text-secondary, #aaa);
-  }
-
-  .paste-new {
-    background: var(--bg-tertiary, #333);
-    color: var(--accent-color, #7aa2f7);
-    border: 1px solid var(--accent-color, #7aa2f7);
-  }
-
-  .paste-send {
-    background: var(--accent-color, #7aa2f7);
-    color: #fff;
-    font-weight: 600;
-  }
+  /* Paste overlay styles extracted to universal section below (line ~2293+) */
 
   /* LEGACY: Hide old toolbar select (no longer used on mobile) */
   .toolbar-select {
@@ -2267,6 +2189,89 @@ html.mobile-init .file-browser-panel {
   background: #3d5f85;
 }
 
+/* ============================================================================
+   Paste Overlay — all mobile/tablet sizes
+   Extracted from phone breakpoint so it applies on iPad too.
+   ============================================================================ */
+.paste-overlay {
+  position: fixed;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.6);
+  z-index: 10000;
+  display: flex;
+  align-items: flex-start;
+  justify-content: center;
+  padding-top: 15vh;
+}
+
+.paste-dialog {
+  background: var(--bg-secondary, #1e1e2e);
+  border: 1px solid var(--border-color, #444);
+  border-radius: 12px;
+  padding: 12px;
+  width: calc(100% - 24px);
+  max-width: 400px;
+}
+
+.paste-textarea {
+  width: 100%;
+  min-height: 80px;
+  max-height: 200px;
+  background: var(--bg-primary, #0d0d14);
+  color: var(--text-primary, #e0e0e0);
+  border: 1px solid var(--border-color, #444);
+  border-radius: 8px;
+  padding: 8px;
+  font-family: inherit;
+  font-size: 16px;
+  resize: none;
+  box-sizing: border-box;
+}
+
+.paste-textarea:focus {
+  outline: none;
+  border-color: var(--accent-color, #7aa2f7);
+}
+
+.paste-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 8px;
+  margin-top: 10px;
+}
+
+.paste-cancel, .paste-new, .paste-send, .paste-image {
+  padding: 8px 18px;
+  border: none;
+  border-radius: 8px;
+  font-size: 14px;
+  cursor: pointer;
+}
+
+.paste-image {
+  margin-right: auto;
+  background: var(--bg-tertiary, #333);
+  color: var(--accent-color, #7aa2f7);
+  border: 1px solid var(--accent-color, #7aa2f7);
+}
+
+.paste-cancel {
+  background: var(--bg-tertiary, #333);
+  color: var(--text-secondary, #aaa);
+}
+
+.paste-new {
+  background: var(--bg-tertiary, #333);
+  color: var(--accent-color, #7aa2f7);
+  border: 1px solid var(--accent-color, #7aa2f7);
+}
+
+.paste-send {
+  background: var(--accent-color, #7aa2f7);
+  color: #fff;
+  font-weight: 600;
+}
+
 /* ============================================================================
    iOS Safari Specific Fixes
    ============================================================================ */