feat: add setting description popovers on hover

[RonenMars]

Adds contextual info popovers to each setting item, displaying the full Ghostty documentation description on hover. This gives users quick access to what each setting does without leaving the app.

Demo

Each setting with a known description gets a small ⓘ icon next to its name. Hovering over it shows a positioned popover with the full description from the Ghostty schema.

How it works

Concern	Implementation
Description lookup	getSettingDescription(id) maps camelCase setting IDs → kebab-case schema keys
Data source	Existing ghostty-schema.ts (auto-generated from Ghostty's Config.zig)
Trigger	Mouse enter/leave with 150ms debounced hide (prevents flicker when moving to popover)
Positioning	Fixed positioning based on icon's getBoundingClientRect(), clamped to viewport
Animation	Svelte scale transition (150ms, starts at 0.95) for smooth appear/disappear

Changes

src/lib/utils/schema.ts (new)

• camelToKebab(id) — converts camelCase setting IDs to kebab-case keys for schema lookup, handling edge cases like x11InstanceName → x11-instance-name and consecutive capitals
• getSettingDescription(id) — looks up a setting's description in the Ghostty schema by ID

src/lib/components/settings/Item.svelte

• Accepts new optional settingId prop
• Renders an info icon (ⓘ SVG) next to the setting name when a description is available
• Shows a fixed-position popover on hover with the full description
• Popover stays visible when hovering over it (for readability), dismisses with 150ms delay
• Viewport-aware positioning prevents popover from overflowing the right edge
• Styled with frosted glass effect (backdrop-filter: blur) matching the app's aesthetic

src/routes/settings/[category]/+page.svelte

• Passes settingId={setting.id} to each <Item> component

Design decisions

• No click required: Hover-based interaction is consistent with native macOS tooltip patterns (matching the app's macOS System Preferences aesthetic)
• Graceful degradation: Settings without a schema description simply don't show the icon — no empty popovers
• 150ms hide delay: Prevents the popover from flickering when the user moves their mouse from the icon to the popover content
• Fixed positioning: Avoids layout shifts and overflow issues within scrollable settings panels