Characters: build the page around Darragh Oakwanderer's sheet

Replaces the Characters placeholder with a real 5e character sheet
feature, following the same read-only-detail/edit-split/game-notes
pattern as NPCs and Enemies:

- Character entity: identity, combat vitals, six ability scores, saving
  throw and skill proficiencies (computed modifiers, not stored), a
  small attacks table, and the sheet's freeform boxes (features,
  equipment, personality, appearance, backstory, allies, treasure).
- Detail page shows the portrait uncropped, then the sheet - genuine
  <table>s for ability scores, skills and attacks, since that's the one
  part of a character sheet that actually is a table; everything else
  reuses the prose helpers from the enemy stat block. Game notes are
  the only editable field here, same as Enemy/NPC.
- Edit page covers every field via CharacterType; the attack rows are
  six plain inputs handled by hand (not a Symfony collection field),
  since it's a short fixed-shape table, not a dynamic list.
- Portraits follow the NPC convention: assets/image/characters/<NameNo
  Spaces>.png, computed path, not a DB column.
- Imported the example sheet from assets/characters/ (Darragh
  Oakwanderer, halfling ranger) directly into the database.

Tests: entity math (ability/save/skill modifiers, passive perception,
portrait path), template rendering for show/edit/index (catches broken
Twig/macro/path()/asset() calls), and the login-gate redirect. 21/21
passing; schema validate clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/enemy/_statblock.html.twig

@@ -1,6 +1,7 @@
 {#
  # Shared 5e stat-block rendering. Used on the enemy detail page and, folded
- # up, inside each tracker combatant.
+ # up, inside each tracker combatant; its prose helpers (mod, _line, _section)
+ # are also borrowed by the character sheet (characters/_sheet.html.twig).
  #   {% import 'enemy/_statblock.html.twig' as sb %}
  #   {{ sb.block(enemy) }}
  #}