feat(themes): add social share buttons to posts

[x3ek]

Closes #12

Share buttons on post pages across all three bundled themes: Twitter/X, LinkedIn, Facebook, Reddit, Hacker News, Email, plus a copy-link button.

• Shared share_urls Jinja filter builds the links from the existing canonical_url with full percent-encoding (unicode titles included); returns empty when site.url is unset so the section hides entirely
• One shared partial (themes/default/_share.html) with inline SVG icons and a few lines of clipboard JS (navigator.clipboard with an execCommand fallback); blue-tech and terminal include it via the loader's default-theme fallback, zero duplication, content repos can override it
• Direct share URLs only, no tracking scripts or widgets; rel="noopener noreferrer" on external links; per-theme CSS matching each theme's look
• 16 tests: platform set and order, encoding edge cases, hidden case, and render tests across all three themes

Browser-verified on all three themes: share row renders in each theme's style, copy button shows its feedback state, links are correctly encoded, and the section is absent when site.url is not configured.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds first-party social sharing to SquishMark’s bundled post themes by introducing a shared Jinja partial and a new share_urls filter that generates fully percent-encoded platform URLs from the render context’s canonical_url. This keeps sharing privacy-friendly (direct links only) while allowing themes/content repos to override the partial if desired.

Changes:

• Add share_urls Jinja filter to generate ordered (platform, url) share links with strict percent-encoding.
• Introduce a shared _share.html partial (default theme) and include it from post.html in all bundled themes via loader fallback.
• Add per-theme CSS for the share row/buttons and add tests covering filter behavior + rendering across themes.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
themes/terminal/static/css/style.css	Adds terminal theme styling for share buttons.
themes/terminal/post.html	Includes shared share partial and sets a terminal-specific label.
themes/default/static/style.css	Adds default theme styling for share buttons.
themes/default/post.html	Includes shared share partial on post pages.
themes/default/_share.html	New shared partial rendering share links + copy-link behavior.
themes/blue-tech/static/style.css	Adds blue-tech theme styling for share buttons.
themes/blue-tech/post.html	Includes shared share partial on post pages.
tests/test_share_urls.py	Adds unit + render tests for share_urls and share section visibility.
src/squishmark/services/theme/filters.py	Implements and registers the share_urls filter.
.claude/skills/theme-creator/references/filters.md	Documents the new filter and how themes include the partial.