feat(web): endpoint handles drag vertically — full 2D tip-aimed targeting

User report after the lifted-handle change: "I cannot go up or down,
just side to side" — a consequence of the line-band pin added with the
lift (the floating finger sits above the text; a band-constrained scan
ignored vertical movement entirely).

The endpoint drag is now full 2D: the finger position (plus the
~15px body-center-to-tip offset) maps to page fractions and the
nearest glyph re-anchors the grabbed end — up/down crosses lines,
side-to-side moves along them, clamped so the extent never inverts.
The boundary rect's own band constraint is gone; sliver rects remain
structurally impossible (rects rebuild from the char-table span as
per-line unions).

Direct-fraction probes verified: (0.4, 0.098) → mid line 1; (0.2,
0.117) → line 2 head; (0.72, 0.117) → line 2 mid — spans cluster into
the right per-line boxes. The dot-listener plumbing (pixels → map →
fractions → scan) verified by instrumented probe on the same load.
Final desktop finger pass pending (emulator input pipeline wedged).

web/src/reader/reader.ts

@@ -1776,7 +1776,10 @@ document.addEventListener("alpine:init", () => {
           });
           dot.addEventListener("pointermove", (e) => {
             if (!dot!.dataset.dragging) return;
-            this.pdfAdjustHandleMove(which, e.clientX, e.clientY);
+            // Aim at the TIP (finger + ~15px, the body-center-to-tip
+            // offset): the body floats above the text, the tip shows
+            // where the boundary actually is.
+            this.pdfAdjustHandleMove(which, e.clientX, e.clientY + 15);
           });
           dot.addEventListener("pointerup", (e) => {
             if (!dot!.dataset.dragging) return;