fix: handle UTF-16 LE/BE encoded files with BOM

[lawrence3699]

Fixes #513

Problem

UTF-16 LE/BE encoded markdown files (common when files are saved by Windows editors or converted from other formats) render as mostly blank content because the raw UTF-16 bytes are passed directly to the markdown renderer.

Fix

Detect UTF-16 byte order marks and convert file content to UTF-8 before rendering. The conversion is applied in all three file-reading paths:

• CLI mode (executeCLI)
• TUI file view (ui.Init)
• Stash file loading (loadLocalMarkdown)

A ToUTF8 utility function handles BOM detection and conversion:

• UTF-16 LE BOM (FF FE): decoded via golang.org/x/text/encoding/unicode
• UTF-16 BE BOM (FE FF): decoded via golang.org/x/text/encoding/unicode
• UTF-8 BOM (EF BB BF): stripped
• No BOM / plain UTF-8: passed through unchanged
• Decode failure: falls back to original bytes

golang.org/x/text is already a dependency (v0.32.0), so no new dependencies are introduced.

Before

$ glow utf16le-readme.md
(mostly blank output)

After

$ glow utf16le-readme.md

  # Hello World

  This is a test.

Validation

• go test ./... — all pass (7 new tests for ToUTF8)
• go vet ./... — clean
• Manual test with UTF-16 LE and BE files with BOM

[Copilot]

Pull request overview

This PR fixes rendering of UTF-16 LE/BE markdown files (with BOM) by converting file bytes to UTF-8 before passing content to the markdown renderer, addressing issue #513.

Changes:

• Add utils.ToUTF8 to detect BOMs (UTF-8/UTF-16 LE/BE) and normalize content to UTF-8.
• Apply UTF-8 normalization across CLI, TUI file view, and stash local file loading paths.
• Add unit tests covering UTF-8 passthrough/BOM stripping and UTF-16 LE/BE decoding.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
utils/utils.go	Introduces ToUTF8 BOM detection + UTF-16→UTF-8 decoding utility.
utils/utils_test.go	Adds test coverage for UTF-8/UTF-16 BOM handling and edge cases (nil/empty, multibyte chars).
ui/ui.go	Normalizes on-disk file bytes via utils.ToUTF8 before frontmatter stripping/rendering.
ui/stash.go	Normalizes locally loaded stash file bytes via utils.ToUTF8 before storing into md.Body.
main.go	Normalizes CLI input bytes via utils.ToUTF8 prior to frontmatter stripping/rendering.

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