Feature: Integrate SBN v2 encoding with base-85 codec and compound history support

[itadrous73]

Integrates the SBN v2 encoder/decoder across the desktop and mobile applications,
replacing all v1 encoding call sites while maintaining full backward compatibility
with existing v1 puzzle strings and saved data. This is tracked by branch WASN01.

What changed

• All encodeToSbn() calls replaced with encodeToSbnV2() (9 call sites)
• All universalImport() calls replaced with universalImportV2() (2 call sites)
• encodeURIComponent() added to desktop URL construction for base-85 URL safety
• sbn.js added to both desktop and mobile script loading
• Mobile service worker updated to cache sbn.js for offline/PWA support
• Service worker VERSION bumped.

Why

SBN v2 provides:

• Base-85 encoding (6.41 bits/char vs 6.00) for ~6% smaller region strings
• 4-cell annotation packing (3⁴=81 ≤ 85) for ~25% smaller annotation strings
• Compound history entries that preserve atomic undo/redo groups — importing a
  shared puzzle where placeStarAndAutoX modified 9 cells now undoes as a single
  step instead of 9 individual steps
• Unlimited grid dimensions (plain decimal vs lookup table)

Backward compatibility

• v1 strings (start with letters, e.g. AA2e...) are auto-detected by
  universalImportV2() and delegated to the existing universalImport()
• v1 strings with numeric dim codes (44–99, i.e. 4×4 through 9×9 boards)
  are disambiguated from v2 via a length check: the main part (before ~) is
  compared against the minimum v2 string length for that candidate dimension.
  Since v2 dim-44+ requires 594+ chars and a v1 4×4 string is at most ~14 chars,
  the gap is unambiguous
• v2 strings (start with digits, e.g. 10B...) are decoded natively
• Old localStorage saves load via v1 fallback; re-saving writes v2 format
• All v1 function definitions and constants (SBN_CODE_TO_DIM_MAP, etc.) are
  kept intact — nothing was removed
• decodeSbn() calls for puzzle file loading are untouched (different contract)