Lightbox settings for new controls

[Darkkal]

Parent Issue

Part of #74 — More controls for media in lightbox

Summary

Add new lightbox-specific settings to the existing /settings page under a new "Lightbox" section on the App Settings tab, placed after "Playback & Media". All other sub-issues in #74 depend on these settings.

New AppSettings Fields

// Added to AppSettings in src/types/settings.ts

// Lightbox: Fit & Zoom
lightboxFitMode: "fitBoth" | "fitWidth" | "fitHeight";
lightboxZoomMin: number;   // Minimum zoom %, e.g. 25
lightboxZoomMax: number;   // Maximum zoom %, e.g. 500
lightboxZoomStep: number;  // Increment per +/- click, e.g. 25

// Lightbox: Controls Visibility
lightboxAutoHideControls: boolean;
lightboxAutoHideDelay: number;  // Seconds before auto-hide, e.g. 3

Proposed Defaults

Setting	Default	Range / Options
lightboxFitMode	"fitBoth"	"fitBoth" | "fitWidth" | "fitHeight"
lightboxZoomMin	25	10–100
lightboxZoomMax	500	100–1000
lightboxZoomStep	25	5–100
lightboxAutoHideControls	false	boolean
lightboxAutoHideDelay	3	1–30 (seconds)

Settings UI

Uses the same UI patterns as existing settings: dropdowns for fit mode, number inputs with min/max validation for zoom bounds and timer, toggle switch for auto-hide (delay input disabled when auto-hide is off).

Acceptance Criteria

• New fields added to AppSettings interface and DEFAULT_SETTINGS
• Settings page renders a "Lightbox" section with all new controls
• Fit mode dropdown with 3 options
• Zoom min/max/step number inputs with range validation
• Auto-hide toggle + delay input (delay disabled when auto-hide is off)
• Settings persist via settings.json and are consumed by Lightbox.tsx
• Schema evolution: new fields gracefully merge with existing settings.json via existing shallow-merge logic in getSettings()

[Darkkal]

Implemented and validated in #132