fix(sync): add the conditional-write switch to the join screen

[rendyuwu]

What

The first-run join screen now renders the same Behaviour block as Settings >
Sync: the "Endpoint honours conditional writes" switch and its off-note on S3,
and the "nothing here to turn on" note on WebDAV.

Why

Closes #3.

The join screen started from DEFAULT_SYNC_CONFIG (cas: false) and rendered
no switch for it, so a device that joined an S3 remote stored cas: false and
ran on unconditional writes until someone opened Settings > Sync.

How

• Settings' Behaviour block moves out of SyncSection.tsx into
  src/settings/components/SyncBehaviour.tsx, beside SyncField, the other
  component both sync forms share. Text, classes, role and aria-label are
  unchanged. The component takes { config, onChange }, and Settings' handler
  still clears the "Saved" flag.
• JoinSyncScreen.tsx renders it between the sync passphrase and the master
  password, writing into the config the join already holds.
• No plumbing change. useJoinSync already sends configSubset(config), which
  carries cas, to sync_join and stores { ...config, enabled: true }, and
  Rust's provider_config already passes cas to S3 and drops it for WebDAV.
• The default stays off, as the issue asks: the app never probes the endpoint,
  so turning it on is the user's call.

Testing

• pnpm exec tsc --noEmit, pnpm run lint:imports, pnpm run format:check,
  pnpm run verify (17/17), with no test edits.

• A Playwright run against pnpm dev, with vault_status, vault_list and
  sync_join stubbed on top of the browser IPC shim, exercised:

  • join on S3: the switch starts off and the off-note shows
  • switch to WebDAV: no switch, and the WebDAV note shows
  • back to S3, turn the switch on: the off-note hides
  • submit the join: sync_join receives config.cas: true, and the stored
    sync config has cas: true, enabled: true
  • open Settings > Sync: the switch loads on, and still turns off and brings
    the note back

• Replacing the join screen's onChange={setConfig} with a no-op turns that run
  red at "switch turns on" and at sync_join (cas was false). Restoring it
  turns it green again.

• pnpm exec tsc --noEmit clean

• Manual smoke-test of the affected feature

• (If you touched src-tauri/) cargo check clean

• (If UI) tested in pnpm tauri:dev (isolated dev data dir)

No changes under src-tauri/. Not tested in pnpm tauri:dev yet, nor against
a real S3 endpoint.

Screenshots / GIFs

None attached. The join screen gains Settings > Sync's Behaviour block as it
already looks there.

Notes for reviewer

• The join screen also gets the "Behaviour" caption, although its other fields
  have none. It keeps the block identical in both forms.
• A WebDAV join shows the WebDAV note, since the whole block is shared.