fix: resolve iOS freezes and expand offline library

[infinityf4p]

Summary

• fix iOS 16 nested search layout recursion and stop overriding native navigation gestures
• move selectable thread/profile text onto the controlled TextKit renderer and add reliable home-tab reselect refresh
• add built-in/custom reader fonts with validated private TTF/OTF storage
• add transactional offline media saves, backup import/export, storage management, and saved-thread reply update tracking
• move font, local media, voice, and storage file I/O away from the main actor

Root cause

On iOS 16, opening a Boolean navigation destination from a view that was already a NavigationStack destination could recurse through SwiftUI trait/layout updates. The app main thread stayed in CA transaction and AttributeGraph layout while memory continuously grew. Old systems now open these nested searches inside an isolated full-screen NavigationStack; iOS 17 and later keep the local push route. Similar navigation destinations and synchronous local-file paths were audited.

Verification

• TiebaPureTests: 831 executed, 8 skipped, 0 failures on iOS 18.3
• iOS 16.4 UI: forum search, repeated thread search, avatar/profile/thread navigation, local save/open, long-press copy
• iPadOS 18.3 UI: repeated title/main-post/subpost selection and both home reselect refresh paths
• prior targeted coverage also passed on iOS 18.3 and iOS 26.5

Fixes #60
Fixes #62
Fixes #63
Fixes #64
Fixes #65