feat(flow-chat): support external file context input

[wgqqqqq]

Summary

• Add one ordered, path-aware intake for external files, directories, and images in the Desktop chat composer.
• Accept multi-item native drag/drop and OS file clipboard paste while preserving internal context drag/drop, bitmap image paste, normal text paste, and large-text capsules.
• Normalize and deduplicate POSIX/Windows paths, classify entries through workspace metadata, keep successful items when part of a batch is invalid, and respect the existing image count limit.
• Fix WebKit's equivalent non-collapsed caret range so the first Backspace removes the adjacent file chip atomically instead of only moving the visible caret.
• Reject controller-local paths for Remote Workspace, Peer Device Mode, Detached Dispatch, and server/web sessions instead of forwarding or silently copying them.

Fixes #1276

Related reports consolidated into this request: #2352 and #2312.

Type and Areas

Type: Feature / regression fix / test / docs

Areas: Desktop/Tauri, Web UI flow chat, interactive capability catalog, E2E

Motivation / Impact

The composer previously understood only BitFun-internal drag payloads, and the main Tauri window disabled native drag/drop handling, so files dragged from Finder/Explorer never reached chat as real paths. Clipboard handling also treated only the first image file specially and otherwise fell back to text.

Desktop users running a local workspace or local managed worktree can now reference repository-external files and directories directly from the composer. No public submission API or generic binary attachment protocol is introduced; non-image files remain existing path contexts.

Verification

• pnpm --dir src/web-ui run test:run src/flow_chat/components/RichTextInput.test.tsx src/flow_chat/utils/externalFileIntake.test.ts src/infrastructure/files/useLocalFileDrop.test.ts — passed, 42/42 tests.
• BITFUN_E2E_APP_MODE=debug pnpm --dir tests/e2e exec wdio run ./config/wdio.conf.ts --spec "./specs/l1-external-file-input.spec.ts" — passed.
• pnpm run check:web — passed.
• pnpm run i18n:audit — passed.
• pnpm run capabilities:generate && pnpm run capabilities:check && pnpm run capabilities:test — passed, 15/15 capability tests.
• pnpm run fmt:rs — passed.
• cargo check -p bitfun-desktop — passed after preparing required generated Desktop resources and configuring the cached sherpa-onnx archive.
• cargo test -p bitfun-desktop clipboard_file_api — passed, 11/11 matching tests.
• macOS Desktop manual check — passed for Finder drag/drop of mixed external items, duplicate handling, and first-Backspace deletion across three file chips.

Reviewer Notes

• Remote behavior is intentionally gated at intake: Remote Workspace, Peer Device Mode, Detached Dispatch, and server/web do not receive controller-local absolute paths.
• Windows and Linux behavior is covered by path/adapter tests but has not been manually exercised on those platforms.
• Development was AI-assisted. The automated suite and available macOS Desktop path were 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.

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