fix(frontend): stop mount config from showing cache limits it never sends (#811) - #818

Summary

Follow-up to triage of #811 ("Internal Rclone mount not correctly purging used storage"). Two UI-side defects that make the rclone VFS cache limits look configured when they are not.

1. The form fabricated limits that were never sent to rclone.
buildRCloneMountFormData used config.rclone.vfs_cache_max_size || "50G" (and the same for vfs_cache_max_age / vfs_cache_poll_interval). If the stored value was empty, the UI displayed 50G while the backend's if != "" guard in pkg/rclonecli/client.go sent no CacheMaxSize at all — and rclone's default is no cap, so the cache directory grows until the disk fills. That is exactly the reported symptom. Now the stored value is shown verbatim, the defaults remain as placeholder hints, and an inline warning appears when cache mode is on with an empty max size.

2. No indication that a remount is required.
vfsOpt is only built in performMount, and no OnConfigChange handler remounts, so edits to the cache limits do nothing to a running mount until stop/start or an app restart. The section now says so.

Not included

Verified during triage and left for separate changes:

• The option plumbing itself is correct — tested the exact mount/mount payload against rclone 1.72.1, and vfs/stats reported CacheMaxSize: 53687091200, CacheMaxAge: 1814400000000000, CachePollInterval: 60000000000. When a mount is created, the limits do reach rclone.
• Auto-remount on config change is not implemented here — an automatic remount drops in-flight streams and deserves its own discussion.
• Several options the UI exposes are never passed to the mount: vfs_cache_min_free_space, vfs_disk_space_total, dir_cache_time, async_read, use_mmap, vfs_fast_fingerprint, vfs_read_chunk_streams, read_only, umask, transfers, timeout, syslog. vfs_cache_min_free_space is the notable one since it reads as a cache limit.

Test plan

• bun run check (biome) clean
• bun run build (tsc -b + vite) green
• Config with vfs_cache_max_size: "" → field renders empty with the warning, not a phantom 50G
• Config with vfs_cache_max_size: 50G → field renders 50G, no warning
• Saving an untouched form does not rewrite the stored cache limits

Refs #811