Add search URL edit modal (colon key)

[Jason-Abbott]

• Add textinput import
• Add modalSearchURL enum value
• Add searchURLInput and searchURLErr fields to Model
• Initialize textinput in New()
• Route non-key messages to textinput when modal active
• Handle : key in handleKey()
• Handle Enter/Esc/typing in handleModalKey()
• Render modal in View()
• Add : to help content
• Build, test, lint

Summary of Changes

Added search URL edit modal to TUI, triggered by : key. Uses bubbles/v2 textinput with validation (must be http(s) URL containing %s). Persists to DB via existing SetSetting.

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