Refactor & standardize modal tabs (#3864)

## Description:

Refactors tab handling out of the individual modal components and into
the base o-modal component. Tabs are now declared by passing tabs,
activeTab, and onTabChange props, and a new named header slot pins
consumer-supplied content above the tabs. This standardizes the modal
tab look.

<img width="1089" height="290" alt="Screenshot 2026-05-06 at 12 17
33 PM"
src="https://github.com/user-attachments/assets/08d5a039-0aef-4aa7-b972-1e43b8723685"
/>

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

src/client/LangSelector.ts

@@ -210,7 +210,6 @@ export class LangSelector extends LitElement {
       "join-lobby-modal",
       "emoji-table",
       "leader-board",
-      "leaderboard-tabs",
       "leaderboard-player-list",
       "leaderboard-clan-table",
       "build-menu",