Add combat HP tracker

- one running tracker (Combatant rows); no encounters to manage
- Enemies list gets a "Track" column (checkbox + count) and an
  "Add to tracker" button; picking N of an enemy makes "Goblin 1..N",
  numbering continues if some are already on the tracker
- /tracker: per-combatant current/max HP with an HP bar, Damage / Heal
  by amount, remove one, "Clear all". HP is clamped to [0, max];
  0 HP shows a "Down" badge. Stats are snapshotted from the Enemy on add.
- "Tracker" nav item; tracker_picker Stimulus controller; app.js gains a
  data-format="pick" cell renderer

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

templates/base.html.twig

@@ -30,6 +30,7 @@
                             { route: 'app_characters', label: 'Characters' },
                             { route: 'app_npcs',      label: "NPC's" },
                             { route: 'app_enemies',   label: 'Enemies' },
+                            { route: 'app_tracker',   label: 'Tracker' },
                             { route: 'app_art',       label: 'Art' },
                             { route: 'app_storyline', label: 'Storyline' },
                         ] %}