fix(theme): support tweakcn registry json imports

[BunsDev]

Summary

• add shadcn registry JSON parsing for tweakcn cssVars.light / cssVars.dark themes
• route the import modal through CSS-or-JSON parsing and allow .json drag/drop
• keep the existing debounced parse and bounded editor behavior that resolves the paste/overflow path on current main

Closes #91.
Closes #92.
Supersedes #99.
Supersedes #106.

Verification

• ./script/check_ai_attribution
• ./script/check_rebrand
• git diff --check
• cargo test -p warp-app themes::tweakcn_import --features local_fs
• cargo check -p warp-app --bin cast-codes --features gui

[Copilot]

Pull request overview

Adds support for importing tweakcn themes from shadcn registry JSON (in addition to the existing CSS export format), and wires the import modal to accept/paste/drag-drop JSON input.

Changes:

• Introduces parse_blocks_or_json and a JSON registry parser that extracts cssVars.light / cssVars.dark OKLCH tokens.
• Refactors OKLCH parsing into shared helpers used by both CSS and JSON paths.
• Updates the import modal to route parsing through the CSS-or-JSON entrypoint and accept .json files via drag/drop.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
app/src/themes/tweakcn_import.rs	Adds registry JSON parsing and shared OKLCH/CSS-decls parsing helpers plus tests.
app/src/settings_view/import_theme_modal.rs	Switches parsing to CSS-or-JSON and extends drag/drop to accept .json.

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