Redesign the native lock screen: clock/avatar header, on-screen keyboard, wrong-password shake

The native lock UI was a flat bordered rectangle with three left-aligned
text lines and no shadow, clock, or identity marker - reported directly
as looking unfinished. Splits the redesign across a new transparent-canvas
header (time, date, circular avatar, username) above a redesigned,
centered password box with a real drop shadow and a dimmed placeholder
prompt, plus a genuine on-screen QWERTY-shaped keyboard with working
Shift/Backspace/Return/Space and real click hit-testing shared with the
render path via one `lock_stack_layout` function, and a damped-sine shake
on a failed attempt.

LockConfig gains show_clock/show_keyboard/avatar_bg, each independently
srd.set-able and documented in a new theme.lock.* section in DEFAULTS.md.
native_lock_render_elements now takes one NativeLockFrame struct instead
of positional buffer arguments now that it composites five optional
layers instead of two.

Full workspace build/test/clippy clean (152 wayland tests, +6 new).

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