Add accounts, presets, live sheet panel and equipment tiles

- Login with registration code, sessions, rate limiting, per-user characters
- Share characters with other users (read-only), admin CLI
- Rules presets with a Presets tab, applied/pending changes with diff in a side sheet
- Equipment as tiles with amounts, starting kit removed
- Rename nav tab to "Character", add "New character" button
- Stop tracking _to_delete/

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HJhXuLD2Z6hv3Wqeq9UL9K

backend/data/rules.json

@@ -21,18 +21,6 @@
     "talents_max": 2,
     "proficiency_bonus": 10,
     "expertise_bonus": 20,
-    "kit": {
-      "weapon_cats": [
-        "melee",
-        "firearm"
-      ],
-      "armor_tiers": [
-        "light"
-      ],
-      "resources_min": 1,
-      "resources_max": 2,
-      "tools": 1
-    },
     "carry_per_strength_mod": 10
   },