impl: fall back to CODER_HEADER_COMMAND when the header command setting is blank

[folbricht-ant]

Same change as coder/jetbrains-coder#600 for the Toolbox plugin: CoderSettingsStore.headerCommand falls back to the CODER_HEADER_COMMAND environment variable when the stored setting is empty, matching the Coder CLI and the VS Code extension (src/settings/headers.ts), and following the existing sshConfigOptions / CODER_SSH_CONFIG_OPTIONS fallback in the same class. An unset variable still yields null, so the default is unchanged; an explicitly configured setting still wins. README settings entry and the ReadOnlyCoderSettings doc updated.

Test: CoderSettingsTest.testHeaderCommand (store / env / precedence), mirroring testSSHConfigOptions.

[folbricht-ant]

Pushed 5345773 to match the wording feedback on coder/jetbrains-coder#600: the headerCommand doc and README no longer mention CODER_URL, and the fallback sentence now reads "When this setting is blank, the CODER_HEADER_COMMAND environment variable is used instead, if set."