feat(reader): shortcuts help modal + desktop edge page-turn zones

Help menu (the reader had a growing shortcut/gesture vocabulary with
no discoverability): a ? topbar button, the '?' key, and F1 open a
glass modal listing navigation, zoom/pan, highlight, and touch
gesture reference — format-aware (fixed-layout/PDF rows appear only
where they apply), Esc closes it first in the dismiss chain.

Desktop edge zones: clickable page-turn strips on the left/right
viewport edges (8% width, 44-72px), desktop only (hover+fine-pointer
media query — touch devices use tap zones, avoiding double paging).
Hovering reveals a chevron arrow and a subtle edge gradient. Zones
disable (pointer-events pass-through) while a fixed-layout page is
zoomed so edge clicks belong to content: panning, selection,
highlight editing. fxZoomed tracks zoom state via the renderer zoom
event, reset/fit actions, and init.

web/static/input.css

@@ -1074,6 +1074,144 @@
     color: #ffffff;
   }
 
+  /* Desktop edge page-turn zones: only on hover-capable fine-pointer
+     devices (touch uses tap zones instead). Arrow + subtle edge gradient
+     appear on hover. */
+  .reader-edge-zone {
+    position: absolute;
+    top: 0;
+    bottom: 0;
+    width: 8%;
+    max-width: 72px;
+    min-width: 44px;
+    z-index: 10;
+    display: none;
+    align-items: center;
+    cursor: pointer;
+  }
+  .reader-edge-zone.left {
+    left: 0;
+    justify-content: flex-start;
+    padding-left: 0.75rem;
+  }
+  .reader-edge-zone.right {
+    right: 0;
+    justify-content: flex-end;
+    padding-right: 0.75rem;
+  }
+  .reader-edge-zone::before {
+    content: "";
+    position: absolute;
+    inset: 0;
+    opacity: 0;
+    transition: opacity 0.15s ease;
+  }
+  .reader-edge-zone.left::before {
+    background: linear-gradient(to right, rgba(255, 255, 255, 0.08), transparent);
+  }
+  .reader-edge-zone.right::before {
+    background: linear-gradient(to left, rgba(255, 255, 255, 0.08), transparent);
+  }
+  .reader-edge-zone .reader-icon {
+    color: #ffffff;
+    filter: drop-shadow(0 1px 3px rgba(0, 0, 0, 0.6));
+    opacity: 0;
+    transition: opacity 0.15s ease;
+  }
+  .reader-edge-zone:hover::before {
+    opacity: 1;
+  }
+  .reader-edge-zone:hover .reader-icon {
+    opacity: 0.85;
+  }
+  @media (hover: hover) and (pointer: fine) {
+    .reader-edge-zone {
+      display: flex;
+    }
+  }
+
+  /* Shortcuts help modal */
+  .reader-help-backdrop {
+    position: fixed;
+    inset: 0;
+    background-color: rgba(0, 0, 0, 0.5);
+    z-index: 70;
+  }
+  .reader-help {
+    position: fixed;
+    top: 50%;
+    left: 50%;
+    transform: translate(-50%, -50%);
+    width: min(34rem, calc(100vw - 2rem));
+    max-height: min(38rem, calc(100vh - 4rem));
+    display: flex;
+    flex-direction: column;
+    background-color: color-mix(in srgb, var(--bg-primary) 90%, transparent);
+    -webkit-backdrop-filter: blur(20px) saturate(1.3);
+    backdrop-filter: blur(20px) saturate(1.3);
+    border: 1px solid color-mix(in srgb, var(--border) 70%, transparent);
+    border-radius: 0.875rem;
+    box-shadow: 0 16px 48px rgba(0, 0, 0, 0.45);
+    z-index: 71;
+  }
+  .reader-help-header {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    padding: 0.875rem 1.25rem;
+    border-bottom: 1px solid var(--border);
+    flex-shrink: 0;
+  }
+  .reader-help-body {
+    overflow-y: auto;
+    padding: 1rem 1.25rem;
+  }
+  .reader-help-section {
+    font-size: 0.7rem;
+    text-transform: uppercase;
+    letter-spacing: 0.08em;
+    color: var(--text-secondary);
+    margin: 1.1rem 0 0.4rem;
+  }
+  .reader-help-body .reader-help-section:first-child {
+    margin-top: 0;
+  }
+  .help-row {
+    display: flex;
+    align-items: baseline;
+    justify-content: space-between;
+    gap: 1rem;
+    padding: 0.3rem 0;
+    font-size: 0.85rem;
+  }
+  .help-row > span:first-child {
+    color: var(--text-primary);
+  }
+  .help-keys {
+    text-align: right;
+    color: var(--text-secondary);
+    font-size: 0.8rem;
+    white-space: nowrap;
+  }
+  .help-note {
+    color: var(--text-secondary);
+    font-size: 0.75rem;
+  }
+  .kbd {
+    display: inline-block;
+    min-width: 1.4rem;
+    text-align: center;
+    padding: 0.05rem 0.4rem;
+    margin: 0 0.1rem;
+    border-radius: 0.3rem;
+    border: 1px solid color-mix(in srgb, var(--border) 80%, transparent);
+    border-bottom-width: 2px;
+    background-color: color-mix(in srgb, currentColor 8%, transparent);
+    font-family: ui-monospace, monospace;
+    font-size: 0.72rem;
+    line-height: 1.2rem;
+  }
+
   /* In-book search */
   .reader-search-bar {
     display: flex;