Adds the standard `Cmd+Shift+V` / `Ctrl+Shift+V` shortcut to paste clipboard content as plain text, skipping HTML and rich content.

**Breaking behavior change:** Previously, `Cmd+Shift+V` toggled paste-at-cursor positioning (added in #4088 for Autodesk). Since the "Paste at cursor" preference (#4066) now covers that use case, this repurposes `Cmd+Shift+V` for plain text paste — the standard shortcut in most apps. Users who relied on `Cmd+Shift+V` for paste-at-cursor should use the preference toggle instead.

- **Preference off:** `Cmd+V` pastes at center, `Cmd+Shift+V` pastes plain text at center
- **Preference on:** `Cmd+V` pastes at cursor, `Cmd+Shift+V` pastes plain text at cursor

Note: depends on #8345 for consistent paste positioning between plain text and rich text paste.

Closes #8183

### Change type

- [x] `feature`

### Test plan

1. Copy rich text from a web page
2. `Cmd+V` — pastes with formatting (existing behavior, unchanged)
3. `Cmd+Shift+V` — pastes as plain text, no HTML formatting
4. Toggle "Paste at cursor" preference and verify both shortcuts respect it
5. Verify that `Cmd+Shift+V` no longer triggers paste-at-cursor positioning

- [ ] Unit tests
- [ ] End to end tests

### Release notes

- Added `Cmd+Shift+V` / `Ctrl+Shift+V` shortcut to paste clipboard content as plain text
- Changed `Cmd+Shift+V` from paste-at-cursor to paste-as-plain-text

### Code changes

| Section        | LOC change  |
| -------------- | ----------- |
| Core code      | +105 / -12  |
| Tests          | +97 / -0    |
| Config/tooling | +1 / -1     |