refactor: centralize matches_key in keybindings and make char matching case-insensitive

[felipecm-br]

Description

Centralizes the duplicated matches_key implementations across all controller modules into a single function in src/config/keybindings.rs, while adding case-insensitive matching for character keys and normalizing shifted punctuation.

Details

• Deduplication: Removed 12 duplicate copies of fn matches_key across controllers (branches, commits, commit_files, diff_mode, files, reflog, remote_branches, remotes, stash, status, tree, and gui/mod.rs), reducing code by 59 lines net.
• Case-Insensitive Character Matching: Handles terminal emulators that report uppercase character codes with modifiers (e.g. Ctrl+L vs Ctrl+l), preventing ignored shortcuts.
• Shifted Punctuation Normalization: Handles terminals reporting shifted punctuation (such as < or >) with KeyModifiers::SHIFT.
• Preserved Semantics: Bare single characters without modifiers distinguish uppercase vs lowercase (e.g. D requires Shift whereas d does not).
• Unit tests verifying case-insensitivity, modifier enforcement, and shift distinctions.

Validation

• cargo test --bin lazygitrs (all 372 tests passed)
• cargo fmt --check