fix: preserve experiment tree viewport

[dajiaohuang]

Preserve the experiment tree’s pan and zoom across tabs, task switches, and reloads using the existing per-task workspace state. Reset the viewport when the effective experiment scope changes, after workspace and experiment data load.

The Rust workspace schema accepts and validates the optional viewport. Only completed user gestures save it, avoiding per-frame writes and persisting automatic fits.

Closes #144

Validation:

• UI production build and typecheck; regenerated embedded assets
• 162 UI tests and dev-slot helper tests
• Localization and style checks
• Rust formatting, Clippy, locked build and tests (815 passed, 2 ignored)
• Browser: pan/zoom across Files, table/tree, tasks, and reload
• Browser: manual and automatic scope changes; fallback-scope reload
• Backend: saved user viewport persists; automatic fit leaves it unset
• User acceptance testing in the running dev slot