fix(desktop): validate team snapshot text before import

[Chessing234]

Refs #5734 — the third of its four ingresses, and the follow-up I offered at the end of #5775.

Stacked on #5775. It uses validate_team_definition_text, which that PR introduces, so its commit is in this branch's history and the diff here is only the 2 files below. Please merge #5775 first, or say the word and I will fold this into it instead.

What happens

build_import_team validated exactly one thing:

let name = snapshot.team.name.trim();
if name.is_empty() {
    return Err("Team snapshot name is empty.".to_string());
}

instructions was then copied straight into the TeamRecord unexamined.

The agent-snapshot path does not work that way — agent_snapshot.rs calls
validate_agent_definition_text on the member's display name and system prompt
and refuses the import with "Snapshot definition is unsafe". So the same
concealed characters were rejected inside a member and accepted in the team
wrapped around them, even though the team's instructions are runtime-layered
into every member deployment.

That asymmetry matters more here than at the other ingresses: a snapshot is a
file from outside this install. Import is the one point at which that text is
reviewed before it can execute.

Fix

One validate_team_definition_text call in build_import_team, using the same
"Team snapshot is unsafe: …" error shape the agent path uses for its own
snapshots.

Placement is load-bearing and already correct: build_import_team runs inside
confirm_team_snapshot_import's Phase 1 — validate (no I/O), before key
minting and before the store lock. A rejected snapshot therefore mints no
keypairs and writes nothing, which is #5734's "prove a failed update leaves no
partial write" for this ingress.

The pre-existing empty-name check stays ahead of it and keeps its own wording, so
that error message does not move.

Evidence

5 new tests in team_snapshot/tests.rs, driving the pure build_import_team:

• a snapshot whose instructions carry U+200B is refused, with the codepoint in
  the message
• a snapshot whose team name carries a bidi override is refused
• an ordinary snapshot still imports, with tabs and newlines preserved in the
  instructions
• a snapshot carrying no instructions still imports
• an empty team name still fails with the original "Team snapshot name is empty."
  message, not the new one

Ran:

• cargo test --manifest-path desktop/src-tauri/Cargo.toml --lib — 2440 passed,
  0 failed, 14 ignored (2435 on the base branch + 5)
• cargo clippy --manifest-path desktop/src-tauri/Cargo.toml --all-targets — no
  warnings, no errors
• cargo fmt --manifest-path desktop/src-tauri/Cargo.toml --all -- --check — clean

Not run locally: the import command end to end. confirm_team_snapshot_import is
a Tauri command needing an AppHandle, so this is verified at the pure function
it calls and by reading the phase ordering around it.

Remaining

That leaves one ingress from #5734: publication. Outbound publish is fed by
the local store, which #5775 and this PR now gate on the way in from every
direction a team can arrive — local edit, relay sync, and file import — so it is
defense in depth rather than an open hole. Happy to add it if you would rather
the contract be closed at the write side too.