feat: add runtime DB switching with SSH remote support

[sparkleholic]

Changes

• Add a DB menu in the sidebar that lets users switch the active database at runtime. No server restart neded. Supports file-based (directory + filename) and in-memory modes, and reloads the schema automatically after applying.
• Add SSH remote mode: mount a remote database directory via sshfs into a local temp path and open it live (read-write, no manual sync). Supports both password auth (via sshpass) and private key file auth. The active SSH config is persisted and pre-populates the modal on re-open; switching away from SSH automatically unmounts the filesystem.
• Add documentation in README covering runtime DB switching and SSH remote mode requirements (macOS: macFUSE + SSHFS; Linux: sshfs package).

Test

• Open DB menu, switch between file and in-memory modes
• Configure SSH remote with password auth (sshpass installed)
• Configure SSH remote with private key auth

[adsharma]

@sparkleholic - have you looked into https://github.com/LadybugDB/bugscope?

These type of features may fit in more naturally there. It also doesn't have the REST API bottleneck. The main use case for this repo is a hard requirement to be browser based.

[adsharma]

• High: src/server/utils/SSHManager.js:75 builds sshfs commands by interpolating user, host, remoteDir,
  privateKeyPath, and port into a shell string. Those values come from the browser via src/server/DBConfig.js:26,
  so this is command injection. Use execFileSync/spawn with argv arrays and validate fields.

• High: src/server/utils/Database.js:205 closes the active DB before validating the new config and before proving
  the replacement can initialize. If the new path is bad or this.init() throws, the server is left with this.db =
  null and an empty pool. The same risk exists after SSH mount in src/server/DBConfig.js:27. This needs rollback
  or “open new DB first, then swap”.