fix: improve scanner accessibility and platform support

[VincenzoImp]

Layer 4 (final) of the follow-up stack announced in #25, rebased onto the updated dev now that #29 has landed.

Summary

• expose barcode scanning only on supported Android/iOS native targets
• latch the first barcode result and stabilize clipboard/URL failure handling
• accept only valid HTTP(S) URLs with a host
• support short screens and large text with scrollable filter controls
• add selected semantics and accessible page-reorder actions
• replace stale transient messages safely and sanitize UI exceptions

Note on flutter_lints

This layer also raises the flutter_lints dev dependency from ^3.0.1 to ^6.0.0, with the corresponding pubspec.lock update. The stricter default rule set passes on the current tree, but it is a repo-wide change rather than an accessibility one — happy to split it into its own PR or drop it if you would rather keep this diff focused.

Verification

• formatter and flutter analyze clean
• barcode, platform-gating, semantics, reorder, layout, and transient-message tests pass
• complete stack: 138 tests pass
• the rebase onto current dev reproduced the verified tree exactly, apart from the 1.3.0 release bump; one conflict in scanner_home_page.dart was a hunk-alignment artifact and was resolved to the verified content