feat(settings): theme every content surface and the window backdrop

[datlechin]

Why

A theme stopped at the editor and the grid. Under Dracula or Nord, trees, diffs, diagrams, value viewers and the window around them stayed system gray.

Change

• Every content view takes the theme: JSON and PHP trees, value and hex viewers, inspector fields, diffs, ER and query plan diagrams, charts, result states, and code fields inside forms and sheets.
• New window.background slot. On macOS 26 and later it sets the window background, and the sidebar, inspector and toolbar glass take it on. Chrome, controls, accent and Settings are never painted.
• Default Light and Dark draw what they drew before, except tree values now use the syntax colors and the result bar drops its dot separator.
• Triggers and ClickHouse Parts are data grids, like Columns.

Verified

Each themed surface has a test that its default matches main and that it follows an override. Guard suites fail any theme read or window color outside the listed files. Default screenshots match main pixel for pixel apart from the dropped dot.

Screenshots

Dracula	Before	After
Main window

Nord:

Structure under Dracula:

Default Dark, unchanged:

Not in this PR

• Middle dots remain in other UI text: query timing, chart toolbar, plugin list, quick switcher, version history.
• The Triggers Enabled column reads YES or NO to VoiceOver; the grid has no per-cell spoken label.
• Docs: docs(customization): document the window background and theme scope docs#17, merge after this.