Automatically fill in single-credential entries

### Checks

- [x] I have read the Wiki, searched the open issues, and still think this is a new feature.

### Explain the problem clearly and succinctly:

#### Background
Single-credential entries can be filled automatically in:
* Firefox: “Autofill logins and passwords” feature (enabled by default). It fills single-credential entries automatically.
- KeePassXC-Browser: The Firefox add-on [KeePassXC-Browser](https://addons.mozilla.org/en-US/firefox/addon/keepassxc-browser/) (not to be confused with KeePassDX) provides an option named **"Automatically fill in single-credential entries"** that mimics Firefox’s built-in “Autofill logins and passwords” feature (enabled by default).
* Android: "Google" is used as the default autofill service in Android by default. It fills single-credential entries automatically.

This behavior allows users to skip the manual selection step when only one matching login exists, improving convenience without compromising security.

#### Problem
Currently, KeePassDX does not have an option to automatically fill single-entry credentials, requiring manual selection every time—even when only one matching entry is available. Users migrating from other managers may find this disruptive to their normal workflow.

### Describe the solution you'd like:

#### Proposal
Implement an **optional setting** under the browser or autofill preferences:

> **Option:** Automatically fill in single-credential entries
> **Sub-label:** Let KeePassDX automatically fill in credentials if it receives only a single entry.

#### Rationale
This option would:
- Provide parity with KeePassXC-Browser’s existing functionality.
- Allow users to achieve a seamless autofill experience without relying on Google’s credential provider or Firefox’s built-in manager.
- Improve efficiency when dealing with single-account websites or apps.

#### References
1. Firefox → about:addons → Webext-keepassxc-browser → Preferences → General → "Automatically fill in single-credential entries"
2. KeePassXC-Browser documentation: https://addons.mozilla.org/en-US/firefox/addon/keepassxc-browser/

#### Implementation Notes
- The feature should be **disabled by default** for safety, with an explanatory tooltip describing the behavior.
- Should work consistently for both web and app autofill contexts (depending on KeePassDX’s integration level).

### Describe alternatives you've considered:

_No response_

### Additional context:

_No response_

Start by locating the browser/autofill preference screens and the single-entry selection flow described in the issue. Confirm how web and app autofill contexts are handled; done when an optional, disabled-by-default setting with explanatory text consistently handles a single returned credential.

Written by the indexing model from the issue text.