Fix snapshot race, delete auth, sparse dd, default disk to 5GB

Snapshot race fix:
- Pre-mark sandbox as "paused" in DB before issuing CreateSnapshot and
  PauseSandbox RPCs, preventing the reconciler from marking it "stopped"
  during the flatten window when the sandbox is gone from the host
  agent's in-memory map but DB still says "running"
- Revert status to "running" on RPC failure
- Check ctx.Err() before writing response to avoid writing to dead
  connections when client disconnects during long snapshot operations

Delete auth fix:
- Block non-admin deletion of platform templates (team_id = all-zeros)
  at DELETE /v1/snapshots/{name} with 403, preventing file deletion
  before the team ownership check fails

Sparse dd:
- Add conv=sparse to dd in FlattenSnapshot so flattened images preserve
  sparseness (~200MB actual vs 5GB logical)

Default disk size:
- Change default disk_size_mb from 20GB to 5GB across migration,
  manager, service, build, and EnsureImageSizes
- Disable split-button dropdown arrow for platform templates in
  dashboard snapshots page (teams cannot delete platform templates)

db/migrations/20260310094104_initial.sql

@@ -144,7 +144,7 @@ CREATE TABLE sandboxes (
     vcpus          INTEGER NOT NULL DEFAULT 1,
     memory_mb      INTEGER NOT NULL DEFAULT 512,
     timeout_sec    INTEGER NOT NULL DEFAULT 300,
-    disk_size_mb   INTEGER NOT NULL DEFAULT 20480,
+    disk_size_mb   INTEGER NOT NULL DEFAULT 5120,
     guest_ip       TEXT NOT NULL DEFAULT '',
     host_ip        TEXT NOT NULL DEFAULT '',
     created_at     TIMESTAMPTZ NOT NULL DEFAULT NOW(),