Global customization: startup route, default sorts, layout, icons, playbar labels, format templates, table columns

[LargeModGames]

Makes nearly every part of the UI configurable via config.yml and the Settings screen.

What's new

• Startup route (behavior.startup_route): open on Home, Recently Played, Podcasts, Discover, Artists, or Saved Albums. The screen's data is fetched at startup so it arrives populated.
• Per-screen default sorting (default_sort_playlist_tracks / _saved_albums / _saved_artists / _recently_played): field or field:desc, validated against each screen's available fields. Recently Played gets real sort state (config-seeded, applied on fetch).
• Layout arrangement: sidebar_position: left|right|hidden (hidden auto-reveals while the sidebar has focus), playbar_position: bottom|top, configurable responsive breakpoints (small_terminal_width/_height). Layout math is unified in core/layout.rs and shared by rendering and mouse hit-testing, so hitboxes match in every arrangement by construction.
• Icons: new fields for the gauge fill, sort arrows, episode-played marker, active-source dot, and list cursor, plus a new Icons Settings tab. Fixed-cell glyphs are width-validated (one terminal column) where table alignment depends on it.
• Playbar control labels (behavior.playbar_control_labels): relabel the clickable playbar buttons; hitboxes resize to fit.
• Format templates (format:): the playbar status line and window title are {key} templates (new core/format.rs, zero deps); defaults reproduce the previous output.
• Table columns (tables:): reorder, remove, rename, and resize the columns of every track/album/podcast table (new tui/ui/columns.rs registry).
• Behavior constants: status_message_ttl_percent, playback_poll_seconds, table_scroll_padding, like_animation_frames.

Safe by default

Structural config errors (unknown sort field, bad template key, invalid column id, too-wide icon, unknown label key) warn and fall back to the built-in default instead of aborting startup, so a config typo can never brick the app. Only YAML syntax errors and the pre-existing numeric range checks remain fatal.

Docs

• docs/configuration.md: full customization guide (valid values per field, template placeholder tables, column ids per table)
• examples/config.example.yml: commented example covering every new section, guarded by a test that runs it through the real loader

Testing

• cargo fmt / clippy -D warnings (slim) clean; full default-features build compiles
• 384 tests pass, including new coverage: layout/hitbox alignment render tests (wide + narrow), degrade-instead-of-error loading, column width cross-validation (zero widths, >100% sums), recoverable missing column id, recently-played sort seeding, example-config round-trip
• Manually smoke-tested: startup route with data, right sidebar + top playbar with mouse clicks, relabeled playbar buttons, custom templates/columns/icons, oversized scroll padding still scrolls, deliberate config typo degrades with a warning