chore(cli): unify error-message capitalization across CLI commands

[inureyes]

Across src/commands/*.rs (non-test), error strings passed to anyhow! split 36 capitalized vs 10 lowercase, so mlxcel detect and a few other commands print differently styled errors than the rest of the CLI.

Evidence

Lowercase sites:

• src/commands/detect.rs:54 and src/commands/detect.rs:58 ("failed to load RT-DETRv2 model: {e}")
• src/commands/models.rs:569
• src/commands/chat.rs:300
• src/commands/generate_falcon_ocr.rs:126 and src/commands/generate_falcon_ocr.rs:415
• src/commands/generate_florence2.rs:80
• src/commands/generate.rs:480, src/commands/generate.rs:518, and src/commands/generate.rs:1210

Compare the majority style at src/commands/chat.rs:603 ("Tokenization failed: {e}").

Suggested fix

Adopt the majority style (capitalized first word) and update the 10 lowercase sites. Pure string edits, no logic change.

Acceptance criteria

• All user-facing anyhow! messages in src/commands start with a capitalized first word
• Tests pass