fix: add spacing between the sender label and message in RadioChat

[JustSamuel]

Summary

`RadioChat.vue`'s `"You:"` / `"Radio:"` label ran directly into the message text with no space at all (see screenshot in the linked report). `AdminChat.vue` already handles this correctly for its own `[Label]` prefix with an `ml-2` on the content -- this just applies the same fix instead of a different one-off, so the two chat views stay visually consistent.

Test plan

• Frontend: `yarn type-check && yarn lint && yarn test` clean, 311/311 (including the existing test that renders a locally-echoed message and asserts its text)
• Live click-through in a browser preview was blocked by an unrelated dev-session WebSocket handshake issue (raw WS connects fine; the chat's own handshake didn't complete) -- looks like leftover state from a long dev session, not something tied to this change. The exact rendering path this touches is covered by an existing passing test.

[github-actions]

Coverage Report for frontend

Status	Category	Percentage	Covered / Total
🔵	Lines	88.35%	1312 / 1485
🔵	Statements	85.91%	1440 / 1676
🔵	Functions	83.3%	454 / 545
🔵	Branches	81.39%	884 / 1086

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
frontend/src/components/RadioChat.vue	91.46%	88.23%	78.94%	94.59%	108-109, 113, 172, 189, 47-59

Generated in workflow #116 for commit a6d83bc by the Vitest Coverage Report Action