feat: add copy/paste layer support

[ert78gb]

Implements copy/paste of layers, as requested in #2692.

• Adds Copy and Paste buttons to the layer header (to the right of the layer button group, left of the help ? icon). The Paste button appears once a layer has been copied.
• Copy stores the current layer in an in-memory "virtual clipboard" (NgRx store), so it can be pasted onto any layer — including in another keymap or on another device.
• Paste overwrites the currently selected layer with the copied layer.

Cross-context safety

Paste is guarded so it can never produce an invalid configuration:

• Macros are re-linked by name (macro names are unique). If the target config has no macro with that name, the key becomes a NoneAction (color preserved).
• Switch-keymap references are kept only if the referenced keymap abbreviation still exists; otherwise the key becomes a NoneAction (color preserved). This avoids serializing a -1 keymap index.
• Switch-layer switch layer actions only copy to base layer and overwrite the corresponding key on the destination layer. Same behaviour as UI.
• Structural merge: modules are matched by id, so a layer copied from a different keyboard/layout (e.g. UHK60 ↔ UHK80, ANSI ↔ ISO) can never desync the target's module structure. Modules without a compatible counterpart keep their existing content.

SwitchLayerAction, keystroke secondary roles, and ConnectionsAction host-connection references are left untouched — they serialize to fixed/valid values and at worst become semantically "dead" keys until the relevant layer/connection exists (no corruption).

Notes

• Partial pastes (some incompatible modules skipped) are intentionally silent.
• No undo for paste yet; a general undo/redo is planned separately and would cover this for free.
• Also fixing the default them handling of the web build

Test plan

• Copy a layer, verify the Paste button appears.
• Paste onto a different layer in the same keymap; verify key actions, colors, and macro/switch-keymap keys are correct.
• Paste into a different keymap; verify macro keys re-link by name and switch-keymap keys survive/clear correctly.
• Copy on one device, paste on another (different layout); verify no structural corruption and config saves/loads cleanly.
• Verify buttons don't appear on the add-keymap preview screen.