Workspace file preview limited to 1MB — should support up to 100MB

[ptone]

Bug

When clicking a file path link in native chat to preview a workspace file, files larger than 1MB are rejected with:

File too large for editing (33.7 MB). Maximum is 1MB.

The limit should be 100MB to support common project artifacts (archives, logs, large source files).

Root cause

`pkg/hub/project_workspace_handlers.go:53`:
```go
const maxEditableFileSize = 1 * 1024 * 1024
```

When the workspace file API is called with `?format=json` (which the chat file preview and workspace editor use), the handler at line 471 checks `info.Size() > maxEditableFileSize` and returns a 400 error.

The file preview flow in chat:

1. User clicks a file path link in a chat message
2. `chat-thread.ts` calls `buildFileApiUrl()` → `GET /api/v1/projects/{id}/workspace/files/{path}?format=json`
3. Backend handler reads the file, checks size against `maxEditableFileSize` (1MB), rejects if too large
4. Error surfaces as red text in the preview dialog

Note: raw downloads (without `?format=json`) have no size limit — the Download button works regardless of file size.

Fix considerations

Raising `maxEditableFileSize` to 100MB affects two flows:

1. Chat file preview (read-only inline view) — the path that's broken
2. Workspace file editor (read-write inline editor) — the file-browser edit button

For the editor flow, 100MB may be excessive for in-browser editing. The fix may need to:

• Raise the limit globally to 100MB (simplest), or
• Separate the preview (read-only) and edit (read-write) limits, or
• Add a `?mode=preview` query param that uses a higher limit than `?format=json` (which implies editable)

The frontend also has its own limits:

• `chat-thread.ts:177`: `PATH_PREVIEW_MAX = 512 * 1024` — files above 512KB show "too large to preview inline" instead of content (but don't error — they show the download button)
• `file-browser.ts:506`: `MAX_EDITABLE_FILE_SIZE = 1 * 1024 * 1024` — hides the edit button for files > 1MB

These frontend limits would also need adjustment for the full 100MB experience.

Files

• `pkg/hub/project_workspace_handlers.go:53,471-472` — the backend 1MB limit and error
• `web/src/components/shared/chat/chat-thread.ts:177,2402-2420` — frontend preview fetch and 512KB preview limit
• `web/src/components/shared/file-browser.ts:506` — frontend editor size limit