Add feature hints for sync, variables, encryption, and chaining

[gschier]

Adds dismissible hints that point people at features they may not have found:

• No sync folder: replaces the sidebar's "Setup FS Sync or Git" menu. Configure opens Workspace Settings and highlights the sync row. Dismissing shows a toast saying where the setting lives.
• Same base URL in 3 or more requests: moves it into a variable.
• Literal secret in an auth password field: moves it into a variable.
• Private environment with secret-looking variables: turns on encryption.
• Successful JSON response: points at response() chaining.

DismissibleBanner gets a tip variant with an X, SettingsList can highlight a row by key, and the banner theme now defines its own subtle border so inline code in banners matches the banner.

Still open: one hint at a time, a setting to turn hints off, and moving field hints into CodeMirror tooltips.