Fix canvas help sizing and toolbar labels from #97 review

[Alexander-Mitrofanov]

The latest review on #97 reports a full-width canvas help dialog at the top left in Edge and asks for recognizable toolbar names in the help. The help component now loads its own stylesheet, so an older cached host stylesheet cannot omit its centering, width limit, or heading styles. Moved, Release, Reset, and Undo use the same border, spacing, and colors as the toolbar buttons while retaining bold section-title text and remaining noninteractive.

Targets issue-74-canvas-interactions, which already includes the earlier #97 fixes from #99 and #100. Addresses #97 (comment).

The reported appearance was reproduced in Microsoft Edge by serving the old component with general dialog CSS lacking the newer help rules: the dialog was 1,721 px wide at (0, 0). With this change it is 540 px wide and centered, including in short windows. The reviewer's exact cache state is unknown. Regression checks cover cached dialog CSS, an independent ViewerPanel host, bounded and centered geometry, matching button appearances, keyboard opening, Escape, Close, and focus restoration.

Validation:

• Architecture and all 467 unit tests pass.
• Complete Chromium 153, Firefox 155, and Microsoft Edge 154.0.4258.62 UI workflows pass on Linux.
• Packed/installed package, native/bundled core contracts, and help/citation checks pass.
• All five visual fixtures match unchanged baselines with zero changed pixels.