v3.4.0

v3.4.0 — Reader polish, Ctrl+F, version display, update checker

Wave 1 — Reader quick wins:
  - Removed redundant scroll-restore pass (kept Render priority only)
  - Extracted magic delay constants (SuppressMirrorMs, LongSuppressMs)
  - Removed orphaned Debug.WriteLine diagnostic dump
  - Merged duplicate Copy Link handlers into shared helper (-70 LOC)
  - Empty state overlay now blocks clicks (IsHitTestVisible=True)

Wave 2 — Ctrl+F find in panes:
  - Find bar at top of Reader (Ctrl+F to open, Escape to close)
  - Dual-pane search: Chinese and English simultaneously
  - Gold highlight on all matches, orange on current match
  - Next/Prev navigation (Enter/Shift+Enter or buttons)
  - Match counter ("N of M"), case-insensitive English, exact CJK

Wave 3 — Version display + startup update check:
  - Version 3.4.0 embedded in csproj, shown in Settings → About
  - On startup (5s delay): checks GitHub Releases API for newer version
  - Green notification bar: "ReadZen vX.Y.Z available — [Download] [Dismiss]"
  - Download opens GitHub release page in browser
  - Silent failure if offline — never blocks startup

Note: hierarchical code tree was already implemented (TagDefinition.ParentId,
TagEditorWindow tree builder, parent dropdown, cycle detection). No changes needed.

Also includes (from v3.3.1):
  - Upgraded markdown renderer (headings, code blocks, links, tables, blockquotes)
  - Avalonia.Headless test harness (890/0/0 — zero skips)
  - CONTRIBUTING.md rewrite
  - ZenUriParser mojibake cleanup

Tests: 890 pass / 0 skip / 0 fail