feat: Add named proxy profiles + picker dropdown in new-download dialog (Related to #165 )

### Preflight checklist

- [x] I searched [existing issues](https://github.com/ja7ad/hydra/issues?q=is%3Aissue) and this has not been proposed already.
- [x] I checked the [README](https://github.com/ja7ad/hydra/blob/main/README.md) and this is not already supported.

### Component

Desktop GUI (Hydra Download Manager)

### Problem

0.4.4 added a per-download proxy override, but it's a single free-text field.
This solves the "override per download" need but not the actual pain point:
having to retype a full proxy address every time you add a download.

Concretely, for anyone running a local SOCKS/HTTP proxy (e.g. a v2ray client
exposing a local port for filtered/restricted networks), there's no way to:
- Save a proxy as a named profile (e.g. "Local v2ray SOCKS") for reuse
- Pick from a dropdown instead of typing an address each time
- Have "recently used" or "favorite" proxies persist across dialog sessions

### Proposed solution

- **Proxy profile manager**: a small settings section to add/edit/delete
proxy profiles, each with a name, address, and type (SOCKS5 / HTTP).
- **New-download dialog**: replace the current free-text field with a
dropdown offering "System default", "No proxy", and the list of saved
profiles, plus an "Add new..." option that opens a small inline form.
- Keep the raw text field available as a fallback/advanced option for
genuine one-off proxies, but make profiles the primary, low-friction path.

### Alternatives considered

_No response_

### Platforms this should cover

- [x] Linux
- [x] macOS
- [x] Windows
- [ ] Android / iOS (via `libhydra`)

### Additional context

_No response_

### Compatibility

- [ ] This changes existing behavior (flags, defaults, output format, or the C ABI).

### Contribution

- [ ] I'm willing to open a pull request implementing this.

Start by tracing the existing per-download proxy override in the desktop GUI and locating the settings and new-download dialog entry points. Then determine how proxy settings persist across sessions and how the dialog represents the current override. Done means reusable named SOCKS5/HTTP profiles, profile management, dropdown choices, an add-new flow, and the raw fallback are supported on the listed desktop platforms.

Written by the indexing model from the issue text.