feat(app): give data lines a fine default width and presets

[nmrtist]

Context

Three code paths authored data strokes at three different widths — 1.0 pt for
spectra and line series, 0.7 pt for contours. On a dense 2D plane those strokes
are thick enough to merge neighbouring contour rings, and the width control that
would fix it sat behind Advanced with no unit and no drag notch.

Double-click-to-reset had a second, unrelated problem. docs/reference/shortcuts.md
documents "Double-click a plot → Reset both axes to full range", but the second
press of the double-click starts a zero-distance box or axis zoom, and the
zoom-completion path consumed the release before the reset could run. Under
Browse Zoom, Alt+drag or on an axis strip the documented shortcut did nothing.

Changes

One default width for every data stroke. plotx-figure exports
DEFAULT_DATA_LINE_WIDTH_PT = 0.5, and LineEncoding, ContourStyle,
Series::line and both AxisTrace constructors adopt it. Point-marker radius,
error bars and the integral overlay keep their own widths — the overlay stays at
1.0 pt so it reads against the finer trace.

Width becomes a setting you can find and use. series.contour.line_width
moves from Advanced to Essential, so it sits beside Lowest level as the
second control that decides what dense data looks like. Both width rows now
declare pt and a 0.05 pt drag notch, carry a tooltip explaining what they
affect, and offer Fine (0.5), Medium (0.75) and Bold (1.25) presets next to the
drag field.

Double-click reset runs on the release that reports it, ahead of the
zoom-completion path, so it works under every tool and on the axis strips. A pan
in flight is committed rather than cancelled, keeping it a separate undo record.

Docs (English and Simplified Chinese) state the new default and unit, describe
the presets, and move contour Line width out of the Advanced list into its own
section.

Test modules were extracted to sibling control_tests.rs and navigation_tests.rs
to stay under the repository's 800-line source limit.

Testing

• cargo pr-check — all 7 stages pass (fmt, source sizes, dependency policy,
  default-frontend build, clippy with warnings denied, tests in both backend
  configurations).
• New coverage: line and contour widths share the fine point schema and the
  Essential tier; both encoding defaults resolve to DEFAULT_DATA_LINE_WIDTH_PT;
  the contour section exposes exactly lowest level and line width as Essential;
  the preset list is pinned. Three navigation tests cover a double-click beating
  a zero-distance box zoom, axis strips resetting only their own axis, and a
  double-click during a pan producing two undo records.
• npm run build in docs/ — 69 pages built clean.