M6-05: Improve viewer accessibility and responsiveness

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L833
Tracking commit: 1ec8105

Scope

Labels: priority:P2, type:feature, area:viewer

• Keyboard support for file picker, panels, tree navigation, and export
  buttons.
• Mobile/tablet layout for inspection.
• Color contrast checks.
• Avoid text overflow in panels.

Acceptance criteria:

• Lighthouse or equivalent accessibility checks pass reasonable
  thresholds.
• Manual keyboard-only smoke test passes.

Definition of done

• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.
• Tests, fixtures, or documentation are added at the level appropriate to the change risk.
• User-facing behavior and limitations remain honest in README/docs/viewer messaging.
• Relevant CI checks pass before the issue is closed.