feat: resize standalone Show1D plots in both dimensions

[bobleesj]

What problem does it solve?

A standalone Show1D plot could change height through its corner handle, but
not width. Add explicit initial/maximum widths and diagonal resizing while
preserving the existing side-panel resize behavior. Pointer-move previews stay
browser-local and animation-frame throttled; dimensions synchronize on release.

What should reviewer(s) do?

Create a standalone Show1D with max_width=900, drag the lower-right corner
diagonally, and use Reset View. Check that export retains the dimensions.
For a plot with a side panel, check that horizontal dragging still reallocates
space between the plot and panel rather than resizing the whole viewer.

Verification

• Focused Plot2D/Show1D Python suite: 41 passed.
• Frontend build: all 11 bundles; typecheck passed; 180 frontend tests passed.
• Installed-wheel Jupyter/Chrome check: standalone plot changed from
  900x292.5 to 740x332.5 CSS pixels through its corner handle.
• Wider Python suite was also run: 1168 passed, 29 skipped, 11 failed.
  Failures concern relocated GPU source-contract paths, an outdated workflow
  test, and a folder-watcher timing assertion; this PR does not change them.
• No scientific arrays or numerical computation are changed.

---

This PR follows the
scikit-package workflow for
reproducible scientific software.

[github-actions]

• Delete bobleesj/quantem.widget:sep-8-show1d-diagonal-resize from its remote if you own it, it is not long-lived, and no other PR uses it.
• Remove the dedicated worktree and local sep-8-show1d-diagonal-resize branch after confirming the worktree is clean.
• Fetch electronmicroscopy/quantem.widget and update local main before starting new work.