Android: Material-3 pass over chat UI and Flutter host

[tiensonqin]

Summary

Material-3 pass over the whole Android (Flutter) surface, driven by a per-screen review against Material conventions. Complements the lui backend fixes in logseq/lui#33 (this branch pins lui to that branch — repin to the merge SHA after it lands).

Notable changes:

• Attachment button was dead on tap — OpenAttachmentPicker set attachment_picker_open, but no view consumed the flag (the menu only opened on long-press via context_menu). For FlutterHost the button now sits in a stack and mounts attachment_picker_menu (dropdown_menu ~anchor:above, File / Camera / Photo / Audio, ~on_dismiss: CloseAttachmentPicker) when the flag is set — same pattern as task_status_picker_dialog. iOS path unchanged.
• composer-asset extension registered for Android/Flutter — the OCaml schema listed only the ios/swiftui profile, so composer_asset_preview created an unregistered extension node on Android and the composer crashed when a draft had assets. Schema now declares android/flutter too, and logseqChatExtensionRegistry registers a Flutter widget (image thumbnail via Image.file, attachment fallback) with the matching fingerprint. app_test.ml fingerprint expectation updated.
• Settings pickers are M3 preference rows — theme/language selects now render as ListTile rows (backend change in lui#33) and menu items carry ~selected_signal so the current choice is checkmarked.
• page_delete_dialog, sync_status_sheet, graph_password_sheet get FlutterHost branches — labelled fields, error text, ghost/destructive/primary action bars; previously they emitted iOS style_class markup that rendered bare on Android.
• Back clears block selection — BackAppNavigation now emits the unselect effect when outliner_selected_block_ids is non-empty instead of popping the nav stack, matching the contextual-action-bar convention on Android (pop-only path unchanged for return_to_app_root).
• Host polish — edge-to-edge + transparent system bars, enableOnBackInvokedCallback, SnackBar on native effect errors (was silent log only), and a styled startup-error screen with Retry (was a bare Text).
• Search field gets TextInputAction.search.

Why

The review (per-screen audit of flutter host, nav shell, outliner/composer, settings/search) found: invisible text/backgrounds from unmapped color tokens (backend), dead tap targets (attachment button), a composer crash on assets, iOS-only dialogs rendered bare, a drawer that couldn't close with back and opened on any swipe (backend), back exiting through block selection, and missing host ergonomics (edge-to-edge, predictive back, error feedback).

Testing

dune build @shared/native/runtest — 716 tests green (includes updated composer-asset fingerprint test). Dart changes not locally compiled (no Flutter toolchain here); the Android e2e CI job builds the APK and exercises the app.

Base: devin/1790251566-headless-drive-e2e (main currently can't build — PR #12 fixes the #main-pin vendored-datascript regression this needs).

Link to Devin session: https://app.devin.ai/sessions/9a91e201984a4d4f96ce3e0f86ac6668
Open in Devin Desktop: https://app.devin.ai/desktop/session/9a91e201984a4d4f96ce3e0f86ac6668?variant=devin
Requested by: @tiensonqin

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring