CommandPalette owns its command strings

[meszmate]

Summary

Better fix for the leak that PR #94 patched in user code: solve it in the library so no caller has to think about it.

The real problem

`CommandPalette` stored `Command` structs verbatim — `id`, `label`, `description`, and `shortcut` were all borrowed slices. That forced every caller to keep the source strings alive for as long as the palette existed, and to free them on shutdown. Easy to get wrong: the `action_system` example leaked 8 shortcut strings on exit until PR #94 added a manual ownership tracker on the user side.

That workaround should never have been the caller's job. Other widgets in the codebase (`Table`, `List`, `Tree`) all clone their content; `CommandPalette` was the odd one out.

Library changes

• `addCommand` and `setCommands` now `dupe` every string into the palette's allocator
• `deinit` frees them
• New `setFromRegistry(*const ActionRegistry)` that wires the registry → palette in one call, formatting bindings as shortcut hints

Example collapses

`action_system.zig` loses ~17 lines of manual `palette_cmds` building plus the ownership workaround, replaced by:

```zig
self.palette.setFromRegistry(&self.registry) catch {};
```

Test plan

• `zig build` passes
• `zig build test` — full suite passes
• `./zig-out/bin/action_system` then `q` — zero `memory address … leaked` reports
• Stress test: open palette, run a command, open again, run another, cancel — zero leaks across multiple set/clear cycles