[Keyboard Manager] Remove modifier key requirement in Remap Shortcuts to support single-key multimedia mapping for FnLock-less keyboards

### Description of the new feature / enhancement

I’m a desktop user with a keyboard that has no FnLock, and Fn + F1 is hardcoded by the keyboard driver to open File Explorer (Win+E), which cannot be changed.
To use function keys as convenient multimedia controls, I tried to set these mappings in PowerToys Keyboard Manager:

- Single press F1 = Volume Mute
- F2 = Volume Down
- F3 = Volume Up
- F9 = Previous Track
- F10 = Play/Pause Media
- F11 = Next Track
- Ctrl + F1~F11 = keep original function key behavior (F1 help, F2 rename, etc.)

But I’m blocked by two strict limitations:

1. Remap Keys works globally and cannot target or exclude apps, soCtrl+F1 also becomes muted — no way to preserve the original function with a modifier.
2. Remap Shortcuts forces all shortcuts to start with a modifier key (Ctrl/Alt/Shift/Win) and does not allow single-key remapping. This makes it impossible to map a single function key directly to a multimedia action while using the "Target App" feature to exclude certain apps.

These restrictions make it very hard for users without FnLock to use function keys as practical multimedia controls while preserving their original F1–F12 functionality with modifiers.

### Scenario when this would be used?

Expected behavior
1. Remove the rule that “shortcut must start with a modifier key” in Remap Shortcuts.
2. Allow users to set a single key (e.g., F1) as the original shortcut.
3. Support mapping single function keys directly to multimedia functions: Volume Mute, Volume Up/Down, Play/Pause, Previous/Next Track.
4. Keep the per-application target feature so users can preserve original function keys when using modifiers like Ctrl.

My real usage scenario
I use a desktop keyboard with no FnLock, and Fn+F1 always opens File Explorer (driver/hardware fixed).
My daily need is very common and simple:
- Single press F1–F11: Control volume and media playback
- Ctrl + F1–F11: Keep the original function key behavior (help, search, etc.)

Right now PowerToys cannot support this basic and widely needed workflow because of the modifier key restriction and global-only remap.
This improvement would make function-key multimedia control much more accessible for desktop users without FnLock.

Suggested UI/UX improvement
In "Remap Shortcuts", currently the "Original Shortcut" field only accepts key combinations starting with modifiers. I suggest removing this validation and allowing any single key (like F1) to be set as the original shortcut. Then, users could map F1 to Volume_Mute and set the target app to ! (all apps except) or a specific exclusion list to achieve the desired behavior.

### Supporting information

- Many desktop keyboards do not have FnLock, and users rely on software remapping to use F1–F12 as multimedia keys.
- Mapping single function keys to multimedia is one of the most frequent needs for Keyboard Manager users.
- The modifier-key-only restriction is unnecessary and blocks a natural, intuitive key mapping workflow.
- Supporting single-key remap in Remap Shortcuts would perfectly solve the conflict between multimedia functions and original function key usage.
- This change would also benefit laptop users who want to customize their Fn layer without relying on manufacturer software.

Start in PowerToys Keyboard Manager's Remap Shortcuts flow, especially validation for the Original Shortcut field and handling of Target App mappings. Confirm how single-key shortcuts and multimedia actions are represented, then ensure single keys such as F1 can be accepted and targeted without removing per-application behavior; the existing Keyboard Manager coverage should pass or be extended if present.

Written by the indexing model from the issue text.