Codebase restructure and cleanup

[nevermore23274]

The codebase has grown significantly through v0.3–v0.8 and several files have accumulated multiple responsibilities. I think some of the functionality from files (specifically App.rs which is over 1k lines) should be removed and thoughtfully placed in a core section to make the code more reusable and readable for maintainability.

Current pain points I can see

• app.rs (1068 lines): holds the App struct, all enums, PerfStats/FrameTiming (~100 lines), three async RadioBrowser fetch functions and FetchResult (~180 lines), NowPlaying, SongLogEntry, and every App method. Too many concerns in one file.
• main.rs (455 lines): the event loop is ~350 lines of inline keybinding dispatch across overlays (theme picker, genre picker, settings, normal mode). Makes it rough to navigate.
• pipe.rs (570 lines): mixes a reusable generic (SeqLock), the FFT implementation, and the FIFO reader/PCM pipeline.
• No core/ module: domain types, perf tracking, and radio API logic live alongside UI state in app.rs.

The Proposed Structure

src/
├── main.rs                   Event loop only (slim)
├── core/
│   ├── mod.rs
│   ├── app.rs                App struct, construction, core methods
│   ├── types.rs              InputMode, ActivePanel, Overlay, QueryKind, NowPlaying, SongLogEntry
│   ├── radio.rs              FetchResult, fetch_blended_by_tag/name, fetch_more, filter_spam
│   └── perf.rs               PerfStats, FrameTiming, PerfSummary
├── audio/
│   ├── mod.rs
│   ├── player.rs             (unchanged)
│   ├── pipe.rs               FIFO reader + PCM pipeline only
│   ├── fft.rs                fft_in_place, compute_band_edges, group_into_bands, compute_band_energies
│   ├── seqlock.rs            Generic SeqLock<T> (reusable)
│   └── visualizer.rs         (unchanged)
├── input/
│   ├── mod.rs
│   └── handler.rs            Keybinding dispatch extracted from main.rs
├── storage/                  (unchanged)
└── ui/                       (unchanged)

Breakdown

Extract core/ module from app.rs

• core/types.rs: move InputMode, ActivePanel, Overlay, QueryKind, NowPlaying, SongLogEntry
• core/radio.rs: move FetchResult, fetch_blended_by_tag, fetch_blended_by_name, fetch_more, filter_spam
• core/perf.rs: move PerfStats, FrameTiming, PerfSummary and their impls
• core/app.rs: what remains: App struct and its methods

Split pipe.rs

• audio/seqlock.rs: The generic SeqLock<T: Copy> with its safety docs
• audio/fft.rs: fft_in_place, compute_band_edges, group_into_bands, compute_band_energies, and the test suite
• audio/pipe.rs: AudioAnalysis, SharedAnalysis, FIFO creation, spawn_reader, reader_loop

Extract input handling from main.rs

• input/handler.rs: keybinding dispatch for each overlay and normal mode
• main.rs: event loop skeleton, terminal setup/teardown, tick timing