Vote Rewards: fix duplicate provider, broadcast tokens, layout

[AdaInTheLab]

Summary

Three UX/correctness fixes spotted during the first real end-to-end test:

1. Duplicate provider entry on every server boot

The /vote chat command was printing two lines for one configured provider:

```
From Server: 7daystodie-servers: provider disabled
From Server: 7daystodie-servers.com: granted 100 points (balance: 100)
```

Cause: Newtonsoft.Json's default behavior with a property whose initializer returns a non-empty collection is to append JSON-loaded entries to the constructor-default list, not replace it. So:

```csharp
public List Providers { get; set; } = new List {
new VoteProviderSettings { Key = "7daystodie-servers", Enabled = false }
};
```

…meant every boot loaded the saved entry on top of the default, doubling the list. Fix: default `Providers` to empty, always run `EnsureDefaultProviders` after deserialization. The duplicate self-resolves on next boot — saved JSON has 1 entry; without the constructor pre-fill, deserialization yields exactly 1.

2. Broadcast hint rendered as "Tokens: , ."

vue-i18n was interpreting the literal `{player}` / `{reward}` in the translation as named-parameter slots, finding no params, and substituting empty strings. Fix: rename slots to `{playerToken}` / `{rewardToken}` and pass the literal token strings as parameters from the template.

3. Provider config was a tall single column

Wrapped each provider's fields in a 2-col CSS grid. Toggle / API key / broadcast template span the full row; Server ID + Poll Interval pair up; Reward Type + reward-value pair up. Mirrors the Discord tab's grid pattern. Collapses to 1 col under 768px.

Files

File	What
`Features/VoteRewardsSettings.cs`	Default `Providers` to empty list + comment explaining why
`Features/VoteRewardsFeature.cs`	Always call `EnsureDefaultProviders` after deserialization (was only inside the persisted-state branch)
`frontend/src/views/SettingsView.vue`	New `vote-provider-grid` 2-col layout + i18n params for broadcast hint + extracted broadcast placeholder const
`frontend/src/i18n/locales/{en,ja,ko,zh-CN,zh-TW}.ts`	`{player}`/`{reward}` → `{playerToken}`/`{rewardToken}` in the broadcast hint

Test plan

• Pull, build, deploy to dev server with rsync
• Hard-refresh panel (`Ctrl+Shift+R`)
• Settings → Vote Rewards: only one provider block visible (was two before)
• Provider fields lay out in 2 columns at desktop width
• Broadcast template hint reads "Tokens: {player}, {reward}. Empty = silent." (was "Tokens: , .")
• `/vote` chat command prints exactly one provider line, not two
• Resize to mobile width → grid collapses cleanly to 1 col

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