fix(ppt-live): inherit model and preserve PDF text

[bobleer]

Summary

• Remove PPT Live's duplicate model selector and raw AI model-catalog permission.
• Keep agent-backed MiniApp sessions synchronized with model and reasoning choices made in the shared floating ChatInput.
• Prefix cached export HTML with a UTF-8 BOM so hidden WebViews decode non-ASCII slide text correctly for PDF and PNG output.

Fixes: N/A

Type and Areas

Type: bug fix, regression fix, UI/UX, test

Areas: PPT Live built-in MiniApp, web UI chat model selection, desktop/Tauri export host

Motivation / Impact

PPT Live now has one model-selection surface: the shared floating chat input used to submit generation requests. Existing saved decks no longer restore a stale MiniApp-owned model preference, and the hidden agent session receives model and reasoning changes from that shared selector.

PDF export no longer relies on charset sniffing for sanitized full-document slide HTML, preventing Chinese and other non-ASCII text from becoming mojibake.

Verification

• pnpm run test:ppt-live — 194 passed
• pnpm --dir src/web-ui run test:run src/flow_chat/components/ModelSelectorExternal.test.tsx — 7 passed
• pnpm run type-check:web — passed
• pnpm --dir src/web-ui exec eslint src/flow_chat/components/ModelSelector.tsx src/flow_chat/utils/modelSelectionTarget.ts — passed
• pnpm run check:repo-hygiene — passed
• cargo test -p bitfun-product-domains --features miniapp ppt_live_bundle_uses_bitfun_host_capabilities — passed
• cargo check -p bitfun-desktop — passed
• cargo test -p bitfun-desktop — 281 passed, 2 ignored
• Manual PDF render QA: a BOM-prefixed full HTML document without a charset meta tag rendered Chinese and café correctly; Poppler text extraction and page rendering both confirmed the output.

Reviewer Notes

• The UTF-8 marker is added at the desktop export-file boundary, after sanitization, so the sanitizer's meta-tag policy remains unchanged.
• The same hidden WebView path serves PDF and PNG slide rendering; both now receive unambiguous UTF-8 input.
• AI-assisted; fully tested.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.