feat(ui): H / F keyboard fits for the plot data viewport

[Limdongcheng]

Summary

H fits the y axis to the data visible in the current x window (the NMR vertical-fit convention — the window-scoped auto range, not full_y) and F fits both axes; both are single undoable viewport steps sharing the double-click reset path.

Plain F was already bound to ZoomToSelection (board-level frame fit), so the chord is context-split by a focused handler instead of the dispatch table: with the pointer on a plot's data area it runs the new FitPlotXY, anywhere else it keeps ZoomToSelection — neither behavior is dropped. H was unbound before. Invoked without a pointer target, both fits fall back to the active plot.

Validation

• cargo pr-check passes (fmt, source sizes, dependency policy, default-frontends build, clippy -D warnings, reference-backend test suite).
• Tests cover the window-scoped y fit, the context split for F, and command gating; shortcuts.rs tests moved to a sibling shortcuts_tests.rs to stay under the source-size limit.

UI display logic

• New or changed actions go through the command catalog (CommandId + describe) and are searchable in the command palette. (FitPlotY / FitPlotXY registered, gated in describe on an active plot, listed in the View menu.)
• Hiding happens only at Ribbon-group level (dataset kind or width budget); transient states disable with a disabled_reason that says how to unblock. (Commands stay visible and disable with a reason when no plot is active.)
• No layout changes from background events; panels are never auto-closed. (Viewport-only changes.)
• New panels or empty regions show an empty state with a next step. (No new panels.)
• Docs updated in docs/ (English and zh-CN) for user-visible behavior. (Keyboard shortcut reference updated in both languages.)