feat(web): PDF highlight extent adjustment — rect-edge grips with word-token snap

The reflowable adjust (objective 9) had no fixed-layout counterpart: the
popover's Adjust button relied on adjustContext, which is only stashed by
the reflowable show-annotation path, so PDF rows showed a button that
silently did nothing. PDF highlights persist as page-fraction rect
payloads ({v:1,page,rects}), so their adjustment is geometry, not range
re-anchoring.

- startAnyAdjust routes the popover button by format; startPdfAdjust
  runs a rect-edge session: 4 grips per rect (all edges, one rect per
  drag), live preview through the renderer's own addRectAnnotation
  (replace-by-key — no fork change), commit via saveHighlightChanges.
- Snap vocabulary mirrors snapBoundary: maximal letter runs are atomic
  tokens (start edge takes the word start, end edge the word end), each
  punctuation glyph is its own token with both edges grabbable (a period
  or line-break hyphen can be taken or shed precisely). Horizontal edges
  snap to line top/bottom bands (own line + neighbours).
- selection_text is recomputed from the glyph boxes inside the final
  rects (reading order, lines joined with spaces) so the stored text
  stays honest with the extent.
- Cancel paths mirror the reflowable session: page turn (renderer
  relocate) discards and restores; a committed session skips the restore.
- getContents() from the fixed-layout renderer carries no index — page
  iframe docs are mapped through a WeakMap filled by the view's load
  events. Pointer mapping goes through the canvas's post-transform rect
  (the dpr denominator lesson) via the fork's own convertPoint ratios.
- pdfCharBoxes dedupes by box key: pdf.js re-renders append a second
  span set into the same .textLayer without clearing, and undeduped
  glyphs doubled the recomputed text.

Device-verified (emulator Chrome + CDP): popover route, grip placement,
word-atomic shrink ("But this" -> "this"), extension to the exact
line-break hyphen boundary (0.8777), far-drag clamp at the last token,
page-turn cancel, and the reflowable adjust still routing to handles.

templates/reader.templ

@@ -166,7 +166,7 @@ templ Reader(user User, metadata ReaderMetadata) {
 							type="button"
 							class="reader-popover-btn"
 							title="Adjust highlight extent"
-							@click="startAdjust()"
+							@click="startAnyAdjust()"
 						>
 							<svg class="reader-icon" width="16" height="16" aria-hidden="true"><path d="M 5 3 v 18 M 19 3 v 18 M 5 8 h 14 M 5 16 h 14"></path></svg>
 						</button>