Add card linking feature

[dinana]

Summary

Adds the ability to link cards together with three relationship types: parent, child, and related. Links are bidirectional - when you link Card A to Card B, both cards show the relationship.

Features

• Link types: Parent (↑), Child (↓), and Related (↔) with distinct arrow icons
• Link picker dialog: Click "Link to another card" to open a searchable dialog with link type tabs
• Server-side search: Type to search cards by title or number
• Bidirectional display: Links appear on both cards with appropriate inverse icons
• Easy removal: Hover over a link to reveal the delete button

Implementation

• CardLink model with source_card, target_card, and link_type enum
• Card::Linkable concern providing parents, children, related_cards relations
• Cards::LinksController with Turbo Stream responses for seamless UX
• Validation prevents self-linking, cross-account linking, and circular parent chains
• Full test coverage (39 tests for the linking feature)

Screenshots

The link section appears below the card title with:

• Chain link icon button to add new links
• Arrow icons indicating relationship direction
• Card badge with number and truncated title

Test plan

• Create a new link using the link picker dialog
• Verify all three link types display correct icons (↑ ↓ ↔)
• Check that links appear on both linked cards
• Delete a link and verify it's removed from both cards
• Search for cards by title and number in the picker
• Verify cross-account linking is prevented