[FEATURE] Accessibility themes and system tray

**Summary:** Add accessibility-focused themes for visual impairments and optional bar/tray integration via low-coupling status output patterns suited to Waybar/Swaybar ecosystems. Related planning: `dev/IMPROVEMENTS/FEATURE_PRIORITY.md` (a11y ~v0.8.0).

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## 1) Accessibility themes

### Problem

Low vision and color-blind users need higher contrast, larger semantic cues, and reduced reliance on color alone.

### Recommended implementation

- **Theme pack:** Additional `theme.conf` presets or named themes: high contrast light/dark, protanopia/deuteranopia-friendly palettes (validate with contrast tools).
- **Tweakables:** optional global spacing/unicode icon substitutions where Ratatui allows.
- **Config:** `theme = high_contrast_dark` etc.; document in example `config/theme.conf` per project rules when implemented.
- **Focus indicators:** ensure focus rings use brightness/symbols, not color-only.

### Acceptance criteria

- Themes pass configurable minimum contrast ratio targets (document ratio in theme docs).
- Help overlay remains readable in all bundled a11y themes.

### Testing

- Snapshot tests of rendered spans where feasible; manual checklist for common terminals.

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## 2) System tray / panel status icons

### Problem

Users want Pacsea presence on Wayland/X11 bars (Waybar, Swaybar, etc.) for quick launch or update status.

### Recommended implementation options

**Option A — External companion (lower coupling)**
Small optional daemon or script emits status via file/socket that Waybar `custom` module reads; Pacsea writes “updates available” when invoked with `--status-json` or similar one-shot.

**Option B — In-process tray (higher effort)**
Rust crate for StatusNotifier / libappindicator; heavy platform/GUI dependency — likely poor fit for pure TUI.

**Recommendation:** Start with **Option A**: extend CLI with machine-readable status flags (pending updates count, last sync error) suitable for bars; provide example Waybar config snippet in repo `contrib/` or wiki when allowed.

### Acceptance criteria

- No tray feature required for core Pacsea; purely optional.
- Status command completes quickly; respects same privilege/tool detection as main app where needed.

### Risks

- Wayland vs X11 tray protocols differ; document limitations.

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Tracking: `dev/ROADMAP/OTHER_accessibility_and_tray.md`