Add tooltips to Win/Loss Score columns in clan stats (#2752)

Resolves #2508

## Description:

Adds hover tooltips to the "Win Score" and "Loss Score" column headers
in the clan stats table to help players understand what these weighted
scores represent.

### Changes Made
- Added tooltip to **Win Score** column: "Weighted wins based on clan
participation and match difficulty"
- Added tooltip to **Loss Score** column: "Weighted losses based on clan
participation and match difficulty"
- Uses native HTML `title` attribute (follows existing codebase
patterns)
- Fully i18n-ready via `translateText()` - other languages will be
translated via Crowdin

### Implementation Details
- **Files Modified**: 2 files, 4 lines total
  - `resources/lang/en.json`: Added 2 tooltip translation keys
- `src/client/StatsModal.ts`: Added `title` attributes to table headers
- No breaking changes

### Expected Behavior
When hovering over "Win Score" or "Loss Score" column headers, users see
a tooltip explaining the weighted scoring system based on clan
participation and match difficulty.

## 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

![correctimplementation](https://github.com/user-attachments/assets/bbbad9ab-36cf-4364-96ff-feff6ee966cd)

src/client/StatsModal.ts

@@ -143,10 +143,16 @@ export class StatsModal extends LitElement {
                 <th class="py-2 px-2 text-right">
                   ${translateText("stats_modal.games")}
                 </th>
-                <th class="py-2 px-2 text-right">
+                <th
+                  class="py-2 px-2 text-right"
+                  title=${translateText("stats_modal.win_score_tooltip")}
+                >
                   ${translateText("stats_modal.win_score")}
                 </th>
-                <th class="py-2 px-2 text-right">
+                <th
+                  class="py-2 px-2 text-right"
+                  title=${translateText("stats_modal.loss_score_tooltip")}
+                >
                   ${translateText("stats_modal.loss_score")}
                 </th>
                 <th class="py-2 pl-2 text-right">