Refuse a sandboxed component whose title is only whitespace

[kevin9327]

What this changes

POST /api/sandboxed took a title of spaces and stored it. The guard checked the field was present,
not that it said anything:

if (!body?.slug || !body.title) { … }
…
  slug: body.slug,
  title: body.title,

The slug is safe: SandboxedStore.save runs it against ^[a-z0-9][a-z0-9_]{0,38}[a-z0-9]$ and
refuses one made of spaces. The title has no check anywhere and is inserted verbatim, so the
component lands in the playground grid with nothing to read, and a Bot granted it is told the
component is called " ".

This is the third endpoint of that exact shape. #376 fixed POST /api/servers/custom and #377 fixed
POST /api/skills yesterday, both by trimming and refusing whitespace; this one was not in either
diff. The change here is the same two lines.

Where it runs

• New state that outlives a request? None. The same row is written, with a trimmed title.
• What happens on the second replica? Nothing differs. The check is per request and reads
  only the body.
• Anything serialised? No change. save still upserts on sandboxedComponents.name.
• Anything fanned out to a browser? No.
• New listener, port, or schedule? None.

Boundary and audit

• Every acting call still goes through the gateway: untouched, this is a save behind
  requireAdmin.
• New refusals and new failures each write a row: the refusal is a 400 on a malformed body, the
  same class the two sibling endpoints answer without an audit row.
• Nothing new is trusted from the client that the server can resolve itself: strictly less is
  trusted than before.

Changelog

• A line in CHANGELOG.md under Unreleased.

Proof

server/tests/sandboxed-routes.test.ts, new. The two sibling fixes shipped without tests, so this
adds the ones that pin the behaviour for this route:

• refuses a title that is only whitespace — 400, and the store is never called.
• refuses a missing title, as it always did — the guard that already worked still works.
• stores a padded title without its padding — " Weather card " is saved as "Weather card".
• leaves an ordinary title alone — no change to the ordinary path.

Before this change, two of the four fail:

(fail) stores a padded title without its padding
- "title": "Weather card",
+ "title": "  Weather card  ",
2 pass, 2 fail

After: 4 pass. bunx tsc --noEmit in server/ is clean, and bunx biome check on both files is
clean.

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