refactor: replace global clippy::pedantic allows with local annotations

[StudentWeis]

Summary

Replace seven blanket clippy::pedantic allows in Cargo.toml with localized #[allow(...)] annotations (each carrying a reason = "..."), and remove a few warnings entirely at the source. After this change cargo clippy --all-targets --all-features -- -D warnings is clean.

Linked Issue

Closes #78

Changes

• Cargo.toml — drop the seven global clippy::pedantic allows so future code is lint-checked again.
• Source-level fixes (no #[allow] left behind):
  • utils::clipboard_files — new hex_digit_value(u8) -> Option<u8> helper keeps percent-decoding in u8 end-to-end, eliminating the u32 -> u8 cast and the misleading try_from clippy suggestion.
  • updater::models — derive Eq on ReleaseInfo (UpdateStatus keeps PartialEq only because it carries an f32 progress value).
  • gui::board::records_list::metrics — add GRID_COLUMN_COUNT_F32 mirror constant so masonry layout math no longer casts at runtime.
• Localized #[allow(... reason = "...")] for the remaining warnings, each documenting the safety invariant at that exact site:
  • gui::board::color — parse_alpha and float_channel_to_u8 are guarded to [0.0, 1.0] before scaling to u8.
  • gui::board::preview — image dimensions from the image crate fit comfortably in f32.
  • gui::board::records_list::metrics / masonry — line counts and column indices are bounded by small compile-time constants.
  • gui::board::settings_editor — opacity slider is bounded by MIN/MAX_OPACITY_PERCENT (0..=100).
  • gui::utils — physical pixel sizes fit in i32 on every supported platform.
  • updater::downloader — download progress only needs sub-percent accuracy.
  • gui::board::actions — gpui::actions! macro can't have Eq injected from outside; the generated items are wrapped in a private submodule scoped with #[allow(derive_partial_eq_without_eq)] and re-exported, so external usage is unchanged.
  • repository::time_index (tests only) — module-level allow for cast lints over small loop indices used in test fixtures.

Testing

• scripts/precheck.sh passes locally (cargo fmt, cargo check, cargo clippy -- -D warnings, cargo test 421 passed, cargo machete, i18n / icons / themes checks)
• No new tests needed — refactor is purely mechanical, no behavior change