fix(web): address xhigh code-review findings for the document reader (#197)

Correctness / a11y:
- Rebuild the reader drawer on Radix Dialog (like KbSettingsSheet) instead of
  a hand-rolled overlay: proper modal focus trap, initial + return focus,
  Escape, and background inert (was: aria-modal with none of it) [#4]. This
  also removes the hand-rolled window keydown listener that re-subscribed every
  render [#7].
- Restructure each document row so the open-reader target is a real <button>
  and the delete control is a SIBLING, not nested. Keyboard-activating delete
  no longer bubbles into opening the reader, and the invalid nested-interactive
  markup is gone [#1, #5].
- Resolve a source file by the doc's own type (long → .json first, else .md),
  so two docs sharing a doc_name each resolve to their own file rather than
  whichever extension is tried first [#2].
- Guard source reads: skip non-dict page entries, reject non-list JSON, and
  return a controlled 500 on corrupt/unreadable sources instead of an
  unhandled exception [#3].
- Invalidate the per-hash content cache when the inventory changes, so a
  reopen after recompile refetches instead of serving stale text [#6].

Fetch/cache/memoized body moved to DocumentsPane so they survive the drawer's
unmount-on-close. #8 (frontmatter stripping) intentionally not applied: source
docs render verbatim (a user's own frontmatter is content, unlike wiki-page OKF
metadata). Adds tests for the collision and malformed-JSON paths.

Author: KylinMountain