Add ChatGPT-style voice transcription and image attachments

[PixelShober]

Goal / Problem statement

• Add ChatGPT-style voice input (microphone capture -> transcription -> insert into chat input) and image attachment flow (upload + thumbnail) aligned with Nele AI API.

Acceptance Criteria (bullet list)

• Microphone button starts/stops recording and transcribes to text via /transcription; text appears in chat input for editing before send.
• Image attachments use /image-attachment upload (png/jpg/webp only) and show a small thumbnail preview like ChatGPT; sending chat includes type=image and content=uploaded path.
• Drag-and-drop overlay warns immediately for unsupported files or too-large audio/image based on API limits.
• All changes include automated tests covering attachment policy, API payload shape, and UI elements.

Scope (in scope / out of scope)

• In scope: WPF UI changes, upload/transcription API integration, attachment allowlist, previews, and tests.
• Out of scope: Server-side changes or Nele API schema changes.

Constraints (perf, security, compatibility)

• No secret logging; avoid blocking UI during transcription/upload; keep WPF responsive.
• Use API limits from Nele docs (image-attachment types: png/jpg/webp; transcription: 200 MB audio formats).

Test expectations (unit/integration/manual)

• Unit tests for attachment validation and API payload.
• UI tests for microphone button, thumbnail preview, and overlay warnings.

Notes / links / screenshots (NO secrets)

• Doc source: Clients/_Research/nele.ai documentation.html

[PixelShober]

Approach:

• Implement image upload flow via /image-attachment and wire attachments to chat payload with type=image + content=path.
• Add microphone capture (NAudio) + /transcription integration; place transcript into input buffer (editable).
• Update drag/drop overlay to warn on invalid types/size per Nele docs and add small thumbnail previews.

Files likely to change:

• Clients/NeleDesktop/MainWindow.xaml(.cs)
• Clients/NeleDesktop/ViewModels/MainViewModel.cs
• Clients/NeleDesktop/Services/NeleApiClient.cs (+ new audio/image helpers)
• Clients/NeleDesktop/Services/AttachmentPolicy.cs
• Tests/NeleDesktop.UITests/*

Risks:

• Audio device permissions and format conversion; async UI responsiveness.
• Image upload failures and attachment mismatch with API.

Test plan:

• dotnet restore
• dotnet build
• dotnet test
• Manual: record short audio -> transcript appears in input; attach png -> thumbnail preview -> send

Note: will add NAudio dependency (approved by user).