chat: adopt lui scoped theme API for semantic colors

[tiensonqin]

Summary

Adopts the new LUI scoped theme API (logseq/lui#49) so the chat palette comes from the wire instead of a host-side color dictionary.

• View.chat_view now wraps the screen tree in Lui_elements.themed ~tokens:chat_theme_tokens ~mode_signal:(Signal.map chat_theme_mode model_source). chat_theme_tokens carries the LogseqThemePolicy palette: mode-dependent colors (background, surface, autocomplete-row-background) use the new adaptive {light,dark} token form picked per backend effective color scheme, while task/flashcard colors are Fixed hex+alpha strings converted 1:1 from the previous Color(...)/opacity(0.12) literals.
• chat_theme_mode maps model.appearance ("system"|"light"|"dark") to a theme_mode signal, so the appearance setting hot-switches every backend's effective scheme with one patch — previously this only reached the native shell.
• LogseqChatRootView: drops the .luiSemanticColors([...]) dictionary; the SwiftUI palette stays for app chrome (.tint, .foregroundStyle, .background) outside the LUI tree. Flutter keeps LogseqChatTheme/themeMode as platform shell theme for the same reason.
• Bumps the three lui pins to the theme commit 5958c0c (opam pin, SPM revision, pubspec ref).

Verified: dune build @shared/native/runtest — 716/716 pass.

Link to Devin session: https://app.devin.ai/sessions/eab2eca00a5649d09fcf48626d9f09ef
Open in Devin Desktop: https://app.devin.ai/desktop/session/eab2eca00a5649d09fcf48626d9f09ef?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

[devin-ai-integration]

E2E-tested on iPhone 17 Pro sim (iOS 26.5) — built this branch (lui pins at 5958c0cd), Cognito sign-in, sync graph over local db-sync, seeded flashcards, pixel-sampled rendered colors. Results are mixed: core theming works, but presented content loses the scoped theme env.

✅ In-tree theming fully works — dark flashcards shows exact #002D38 page bg, themed surface card (#F8F8F8/#19394D), and flashcard-* tinted rating buttons:

✅ Appearance switch works live both directions (Light↔Dark↔System, no restart; teal #002D38 renders while device stays light — pixel-verified).

❌ .sheet content loses luiSemanticColors — settings sheet renders secondarySystemGroupedBackground #1C1C1E instead of #002D38/#19394D (cards request ~background:"surface"). Likely cause: .sheet is attached at LUISwiftUIRoot.rootContent outside the LUIThemeScopeModifier env scope (LUISwiftUIRoot.swift:91 vs :542); removing the host-root .luiSemanticColors dict broke what previously reached sheets. Add-graph sheet is also unthemed (#F2F2F8).

❌ task-* tokens have no visible consumer on iOS — the composer status picker renders icon-free pills with uniform accent text (the only task-* consumer on Apple is the picker menu-item icon foreground, view_composer.ml:184; icons don't render in this dropdown presentation):

Full check results + more screenshots

• Light bg ≈#FCFCFC, dark bg exact #002D38; System mode follows device (simctl ui appearance). ChangeAppearance also persists logseq.appearance which drives the native root .preferredColorScheme, so the flip isn't isolated to the wire theme-mode patch — but themed colors prove the theme tokens apply.
• Add-graph sheet unthemed (#F2F2F8):
• Light flashcard tints + surface card:
• Dark graph picker after choosing Dark (device stays light):
• Screen recording and pixel-sample data available in the Devin session.