feat(web): highlight extent adjustment — draggable handles with word/punctuation snap

Objective 9 (web half). The selection popover's edit mode gains an
Adjust button: the overlay's show-annotation event stashes the live DOM
range, start/end teardrop handles render over it (host layer above the
iframe), and pointer drags map into the iframe's caretRangeFromPoint.
Snapping: land next to punctuation → the boundary sits exactly there;
land inside a word → the word's edge in the drag direction; whitespace
trims toward the adjacent word. A live preview repaints through the
overlayer under a scratch key; relocation (page turn) cancels safely.

Release recomputes the range CFI + collapsed-end CFI (view.getCFI) and
saves through the popover's own saveHighlightChanges path (PUT +
remove-old/add-new overlay dance), keeping selection_text and the
anchors in lockstep.

Verified live E2E in Brave: extent extended by one word with the
comma/space snap behavior, saved to the server row, handles cleaned;
all snap paths unit-exercised in-page (word end/start, punctuation
exact on both sides, whitespace trim); the test highlight was restored
to its original extent through the same adjust flow (punctuation-precision
drag back to the comma).

web/static/input.css

@@ -945,6 +945,29 @@
   .reader-popover-btn.danger:hover {
     background-color: rgba(153, 27, 27, 0.6);
   }
+  /* Highlight extent-adjustment handles (objective 9): teardrop dots that
+     sit on the range boundary; the stem points at the text line. */
+  .adjust-handle {
+    position: fixed;
+    width: 26px;
+    height: 26px;
+    border-radius: 50% 50% 50% 0;
+    transform: rotate(-45deg);
+    background: #3b82f6;
+    border: 2px solid #ffffff;
+    box-shadow: 0 1px 4px rgba(0, 0, 0, 0.4);
+    pointer-events: auto;
+    cursor: grab;
+    touch-action: none;
+  }
+  .adjust-handle[data-which="end"] {
+    border-radius: 50% 50% 0 50%;
+    transform: rotate(135deg);
+  }
+  .adjust-handle[data-dragging] {
+    cursor: grabbing;
+    scale: 1.15;
+  }
   .reader-note-input {
     width: 100%;
     resize: vertical;