Fix website reflow, text spacing, and keyboard focus

[robgruen]

Summary

• Replace the absolutely positioned, small-screen-hidden copy button with a wrapping layout that keeps the install command readable and the copy action available.
• Add shared keyboard-focus outlines with sufficient contrast in normal and forced-color modes, including navigation links, buttons, selects, and scrollable documentation code blocks.
• Correct the copy button's accessible name and report clipboard failures instead of displaying success when the Clipboard API is unavailable.
• Document the website build and accessibility verification checklist.

Validation

• Website build: npm --prefix site run build.
• JavaScript syntax and DOM type checks passed.
• Ten browser layout scenarios passed, including an exact 320 x 256 CSS-pixel viewport, WCAG text-spacing overrides, and forced colors; no command/button overlap or horizontal overflow.
• 216 real keyboard Tab stops passed across seven homepage/documentation scenarios, with all expected controls reachable.
• Minimum measured focus contrast was 6.74:1 in normal colors and 21:1 in light/dark forced-color emulation, exceeding the 3:1 requirement.
• Native Enter/Space activation passed with a stubbed clipboard, including the exact command payload, unavailable/rejected clipboard access, and feedback reset. These checks also passed at 320 x 256 with text spacing.
• With JavaScript disabled, the inactive copy button is hidden and the install command remains readable.

Exact Windows Aquatic/Desert themes still need a manual check; browser forced-color emulation was used for automated verification.

Linked issues

Closes #229
Closes #230
Closes #231
Closes #232