fix: allow selecting custom fonts

[infinityf4p]

Summary

• resolve TTF and OTF picker filters from their actual filename-based data types instead of the generic font type
• share the supported extension policy with the existing validated font importer
• clarify that built-in rounded, serif, and monospaced designs affect supported Western glyphs, while Chinese typography requires an imported font with Chinese glyphs

Verification

• iOS 16.4 targeted font regressions: 3 passed, 0 failed
• iOS 26.5 unit suite: 835 executed, 6 skipped, 0 failed
• iOS 26.5 reading-settings UI regression: 1 passed, 0 failed
• all test simulators shut down after use

A local iOS 27 runtime is not installed. The picker regression is covered by asserting that each allowed type exactly matches the system-resolved TTF/OTF type on iOS 16.4 and iOS 26.5.

Fixes #75