Improve widget footer: AI disclaimer and compact layout

[neuromechanist]

Summary

• Add configurable AI disclaimer above the footer: "This is a multi-agent AI assistant and may make mistakes. Please verify responses." (faded orange background)
• Merge the two separate footer lines (page context checkbox + powered by) into a single compact row with pipe separator
• Left side: checkbox for "Share page URL to help answer questions"
• Right side: "Powered by OSA v0.7.1 (71a2511)" with OSA hyperlinked to website
• All disclaimer options configurable: disclaimerEnabled, disclaimerText, disclaimerColor, disclaimerBackground
• Color values validated with CSS color pattern before applying (matches existing themeColor approach)

Closes #245

Test plan

• Verify AI disclaimer renders above the footer with orange background
• Verify combined footer shows checkbox on left, powered-by on right, pipe separator between
• Verify "OSA" link opens osc.earth/osa in new tab
• Verify version and commit hash still display correctly
• Verify page context checkbox toggle still works
• Verify layout correct when allowPageContext is false (checkbox and separator hidden)
• Verify disclaimer hidden when disclaimerEnabled is false
• Verify invalid color values fall back to defaults (CSS custom property fallback)

[github-actions]

Preview Deployment

Name	Link
Preview URL	https://feature-issue-245-widget-foo-demo.osc.earth
Branch	feature/issue-245-widget-footer
Commit	fab591d

This preview will be updated automatically when you push new commits.

[neuromechanist]

PR Review Findings

Reviewed by 3 specialized agents: code-reviewer, silent-failure-hunter, code-simplifier.

Critical (1) -- Fixed

#	Issue	Fix
1	CSS injection via inline style attributes: disclaimerColor and disclaimerBackground were interpolated directly into style= without sanitization, allowing arbitrary CSS injection by malicious embedders	Replaced inline styles with validated CSS custom properties (--osa-disclaimer-color, --osa-disclaimer-bg). Color values validated against CSS color pattern before applying, matching existing themeColor approach. Defaults provided via var() fallback.

Important (3) -- Fixed

#	Issue	Fix
2	Dead CSS: .osa-chat-footer { display: none } left as vestige instead of removed	Removed the entire rule block
3	Inline styles instead of CSS custom properties: Disclaimer colors should use the same pattern as themeColor	Colors now applied via container.style.setProperty() with validation in applyWidgetConfig()
4	null/undefined in disclaimer config: Passing disclaimerText: null renders literal "null"; passing disclaimerColor: null produces invalid CSS silently	Added disclaimerText || '' fallback; color values validated before applying, invalid values silently fall back to CSS defaults

Nice-to-have (3) -- 2 Fixed, 1 Documented

#	Issue	Status
5	Separator span unnecessary: A <span> element with duplicated allowPageContext conditional could be replaced with ::after pseudo-element	Fixed -- replaced with ::after on .osa-page-context-toggle, eliminating extra element and conditional
6	PR description mismatch: Said "below the header" but disclaimer is above the footer	Fixed -- updated PR description
7	Separator visibility not updated on dynamic config change: If allowPageContext changes after render, separator stays stale	Pre-existing issue (checkbox had same problem before this PR). Now moot since ::after is tied to the toggle element itself.

All critical and important issues addressed in commit f37c380.