feat: add regex mode to search (#209)

[matheuswhite]

Closes #209.

Problem

Search mode only matched raw substrings, so patterns (digit runs, alternations, anchored matches, …) couldn't be found.

Solution

A regex toggle is added to search mode:

• Ctrl+E toggles regex on/off (mirrors the existing Ctrl+W case toggle).
• A new box in the search bar shows the state next to the case box: [.*] when on, [ ] when off — e.g. [Aa][.*][1/4] Search Mode.
• When on, each line is matched with a regular expression instead of a literal substring. Case sensitivity is folded into the compiled regex (RegexBuilder::case_insensitive), so the case toggle and regex toggle compose.
• An invalid or in-progress pattern simply matches nothing; the existing red "0 matches" bar ([--/--]) signals it — no log spam.

A note on the keybinding

The issue proposed Ctrl+Shift+F, but control combos collapse Shift on most terminals — without the Kitty keyboard protocol, Ctrl+Shift+F is indistinguishable from Ctrl+F and the toggle would silently never fire. Ctrl+E was chosen instead: reliable on every terminal, works in headless-adjacent decoders, and follows the same pattern as Ctrl+W.

Implementation

The two previously-independent, substring-only match computations — the navigation entries built in update_search_state and the highlighted spans built in search_line — now share a single SearchMatcher stored in ScreenMode::Search:

• Compiled once per search change, not per rendered line.
• Returns character-offset match positions (start, len), so the navigation entries and highlight columns stay aligned (the "current" match's black-on-yellow style depends on that alignment).
• Matches on the whole decoded, ANSI-stripped line, so regex anchors (^, $, \b) behave against real line boundaries — the old tail-based to_special_char closure would have mis-anchored them.
• Skips zero-width matches (e.g. a trailing .*) so they don't inflate the count with nothing to highlight.

is_regex is threaded from InputsShared through the three update_search_state call sites, ChangeToSearchMode, and the search-bar renderer, following the existing is_case_sensitive path exactly.

Testing

• 9 unit tests on SearchMatcher (screen.rs): plain case-sensitive/insensitive, char (not byte) columns for multi-byte input, regex digit-runs, case-insensitive flag, ^ anchoring, zero-width-match skipping, invalid regex → no match, empty query.
• New e2e test regex_search_toggles_with_ctrl_e_and_matches_each_line (tests/tui_e2e.rs): literal \d+ matches nothing → Ctrl+E → two digit runs match, box reads [.*].
• Full suite green: 194 unit, 12 e2e (1 pre-existing macOS-only ignore), cargo fmt clean, no new clippy lints.
• Verified live in the TUI: toggle + box, per-line regex highlighting (current match emphasized), Ctrl+W×regex interaction, ^ anchoring, invalid/no-match red bar, and match navigation (Up/Down).

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