feat: add backlinks panel

[SulimanAbdulrazzaq]

Summary

• add a Backlinks panel for notes that reference the current file
• recognize exact sibling wikilinks, aliases/headings, and relative Markdown links while excluding image embeds and prefix matches
• bound the recursive scan using the existing global-search limits and validate paths in Rust
• open each result at its matching source line

Testing

• node node_modules\vitest\vitest.mjs run src\components\BacklinksPanel.test.tsx --pool=threads --maxWorkers=1 (2 tests)
• node node_modules\typescript\bin\tsc --noEmit
• node node_modules\vite\bin\vite.js build
• rustfmt --edition 2021 --check src-tauri\src\commands.rs
• git diff --check

The focused Rust test could not be compiled locally because this Windows host does not have MSVC's link.exe; CI will provide the Rust toolchain validation.

Closes #89