Add explanation tooltips for derived values

Hover/focus/tap tooltips show how attributes, derived stats, skill target
numbers and limits, capacity, carry weight and point pools are calculated.
Used in the wizard pages, the side sheet and the summary.

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

README.md

@@ -75,6 +75,8 @@ cd frontend && npm install && npm run dev
 
 Characters are saved in your account ("My characters" page). Only the owner can see a character, change it or delete it. On "My characters" the **Share** button lets you add usernames (e.g. your DM): they get read-only access, see the character under "Shared with me" and can open it through `/#creator/<ID>`. Saving a viewed character creates their own copy. Use the JSON backup (Summary page) to move a character between servers.
 
+Hover (or tap / focus) the dotted-underlined labels and the small "i" icons to see where a number comes from, e.g. how a skill's target number and maximum are derived, or what a derived stat is made of. The explanations are generated from the rules, so they follow presets and `rules.json` changes.
+
 While creating, the full character sheet stays visible next to the wizard. Edits are shown as a difference (old → new, added / removed items) until you press **Apply** (or **Apply & next**); **Discard** drops them.
 
 Each wizard page has a **Reset this page** button that clears only that page's data, plus **Start over** for the whole character.