[Feature]: An action that holds the modifiers and taps the main key once, for Cmd+Tab / Alt+Tab style switchers

### Pre-flight checklist

- [x] I searched existing issues and the [Roadmap](https://github.com/AprilNEA/OpenLogi#roadmap), and this isn't already tracked.

### Problem / motivation

Binding a mouse button to the application switcher does not work today, and the two existing shortcut actions each fail it in a different way:

| action | modifiers | main key | result with `Cmd+Tab` |
|---|---|---|---|
| `CustomShortcut` | tapped | tapped | switcher flashes and closes; Cmd is not held |
| `HoldShortcut` (#960, v0.8.0) | held | **held** | switcher runs to the end of the list and parks there |
| `HoldModifier(u8)` proposed in #652 | held | none | nothing taps Tab |
| **missing** | **held** | **tapped once** | what the switcher actually needs |

The switcher UI needs the modifier to stay down (so the overlay stays open and releasing commits the choice) while the main key is tapped exactly once. `HoldShortcut` was built for push-to-talk (#399), where holding the whole chord is the correct behaviour; this is not a bug in it, just a shape it does not cover.

Measured on an MX Master 3 (macOS 15.7.3), listening on `kCGSessionEventTap`, holding the bound button for about three seconds:

| | Cmd held | Tab |
|---|---|---|
| `HoldShortcut` bound to `Cmd+Tab` | 2683 ms | **held the whole time** |
| the same chord typed on a keyboard | 3143 ms | tapped, 5 ms |

### Proposed solution

A new `Action` variant that holds the combo's modifiers for the lifetime of the button press and taps the combo's main key once at press time: the third leg of the family `CustomShortcut` and `HoldShortcut` already form.

Sketch, following the existing conventions:

- `Action::TapKeyHoldingModifiers(KeyCombo)`, appended at the end of the enum. The `config.toml` side is position-independent (variant names are the external tags), but the IPC side encodes bincode variant indices, so appending is the append-only recipe v28 already followed when `Action::HoldShortcut` was added, together with the `PROTOCOL_VERSION` bump and the `cargo test -p openlogi-ipc --test wire_format` update that implies.
- Reuses the press lifecycle `HoldShortcut` already has (`try_hidpp_button_down` / `try_hidpp_button_up`), so no new plumbing.
- `crates/openlogi-inject/src/inject.rs::held_keys()` currently pushes the main key into the held list unconditionally (`keys.push(HeldKey::Key(combo.key()))`); the new variant would hold only the modifiers and emit a balanced down/up for the key.
- Platform-neutral, like every other `Action`: `KeyCombo` already stores abstract modifier bits and the injection layer maps them per platform, so a Windows user picks `Alt+Tab` (the Option bit) and a macOS user picks `Cmd+Tab`. No `cfg(target_os)` in the enum.
- The GUI label can be generated from the combo the way `HoldShortcut`'s `Hold {combo}` label already is, for example `Hold ⌘, tap Tab`.

**Scope, honestly.** This covers one family of interactions: UIs that stay open while a modifier is held and commit on release. On macOS that is Cmd+Tab and Cmd+\`; on Windows and Linux, Alt+Tab; in some editors, Ctrl+Tab for the recent-files switcher. Outside that family I cannot name a use for it. It is a narrower action than `HoldShortcut`, not a more general one.

### Alternatives considered

- **Fixing `HoldShortcut` to tap the key:** no. Push-to-talk (#399, the feature it was built for) needs the key held; changing it would break that.
- **`HoldModifier(u8)` from #652:** solves half of it; nothing taps the key.
- **Sending the chord repeatedly:** advances the switcher on its own and does not keep the overlay open.

### Related area(s)

- [x] GUI
- [ ] CLI
- [x] Button actions / remapping
- [ ] DPI
- [ ] SmartShift
- [ ] Per-application profiles
- [x] Configuration (TOML)
- [ ] Auto-update
- [ ] Other

### Additional context

The `Action` hold axis is already being extended one special case at a time: `HoldShortcut` (#960, merged), `HoldModifier` (#652, proposed), `HoldMouseButton6` (PR #1055, open), hold-to-repeat over HID++ (PR #467, open), plus #1046 and #1054 asking for held mouse buttons for CAD orbit. This is the keyboard-chord leg of the same axis.

It would also close a request that is currently filed away. #792 asked:

> add the ability to setup a custom shortcut for example cmd+ tab o something else

and was closed as a duplicate of #573. #573 is about shortcuts not being assignable to mouse buttons at all, and #1018 is about assigned shortcuts not firing; but even with both of those fixed, `Cmd+Tab` still will not work, because no existing action can express the hold-and-tap shape. As far as I can tell nobody has pointed that out yet.

Start with the Action enum and the existing HoldShortcut press lifecycle, then inspect crates/openlogi-inject/src/inject.rs::held_keys() for how held modifiers and the main key are emitted. Update the IPC protocol and GUI label following the existing conventions, and run cargo test -p openlogi-ipc --test wire_format; done means the new action holds modifiers while tapping the main key once and releases both cleanly.

Written by the indexing model from the issue text.