feat: keep user configurations in sync between computers via a shared file (#3058)

[mondalaci]

Implements #3058.

Adds a Configuration file section to Settings that lets two computers share one keyboard configuration through a file in a synced folder (Dropbox, Google Drive, a network share, ...).

Behavior

• Choose a shared configuration file with a native file picker, or reset to the default path. The row always shows the path, greyed with (default path) / (custom path).
• Detect configuration changes made on the other computer and prompt to apply them checkbox. It is only enabled (and can only be checked) once a custom path is set.
• Agent exports the current configuration to the shared file when it is saved to the keyboard.
• On a detected change, a top notification panel (same design as the dongle-pairing / firmware panels) shows the full path with Load it / ignore actions, instead of a native popup.
• "Load it" goes through the normal import path (autoSave: false), so the config is previewed and the usual Save to keyboard progress button appears.

Detecting changes while Agent is not running

The acknowledged content hash (SHA-256) is persisted (shared-config-acknowledged-content-hash). On startup the file is compared against it, so a change made while Agent was not running is still offered. Applying or ignoring records acknowledgement, so the same content is not offered again (including after a restart) until the file changes. Agent's own exports are also marked acknowledged.

Watching

fs.watch on the file's directory plus a 3 s polling fallback (some sync clients and network shares don't emit reliable events). Bursts are coalesced without starving the check, and a generation guard prevents a stale configure() from re-enabling watching.

Notes

• The file-picker support comes from the already-merged Remember import/export folder #3060 (FileDialogService).
• Verified: uhk-common build, uhk-agent webpack + eslint, uhk-web renderer build + ng lint + unit tests all pass.