feat(erd): fit to screen, zoom controls, table search, click to select #1154

Problem

On a schema with more than a handful of tables the diagram is hard to use:

• It opens at the top-left corner at 100 %; there is no Fit to screen, no zoom buttons and no zoom percentage. Zoom works only with the wheel / trackpad.
• There is no way to find a table on the canvas.
• Relations are highlighted only while the mouse is over a card; moving to read an edge loses the highlight.

Scope

• Canvas controls (bottom-right overlay, QueryaIconButton dense): zoom out, zoom %, zoom in, Fit. Fit on first load. Shortcuts: Ctrl/Cmd + + / - / 0 (fit).
• Search (QueryaSearchField, Ctrl/Cmd+F while the diagram has focus): matching tables listed; Enter centres and selects the table.
• Selection: click selects a table (pinned highlight), unrelated cards and edges are dimmed; Esc or a click on the empty canvas clears it. Hover keeps working when nothing is selected.

Acceptance

• First load fits the whole diagram into the viewport.
• Search + Enter centres the table and selects it.
• A selected table keeps its highlight when the mouse leaves; Esc clears.
• Widget tests for fit, search, select / clear.