feat: add 4-level grayscale display calibration

[markomitranic]

Why

The UC8276 panel can render 4 gray levels, but only the two middle grays are tunable — black and white are fixed. Getting those two mids right is a per-panel, by-eye judgement, and there was no way to do it on the device. This adds a Settings → Calibrate Display screen to dial them in and remember the result.

Depends on protolux-electronics/eink#12. The screen needs both the 4-gray drawing pipeline and the runtime EInk.set_waveform/2 override API, which live in that PR. eink is pinned to its head commit here — this can't merge until #12 merges and a release is cut, after which the dep flips back to the packaged/hex version. Every hardware call is guarded, so it compiles and the simulator runs even before then.

* The test photo is Kodak "kodim17" from the [Kodak Lossless True Color Image Suite](https://r0k.us/graphics/kodak/)

What

A two-step wizard, because two buttons can only tune one gray at a time:

Step 1 — Dark gray    short A/B nudge · hold A = next
Step 2 — Light gray   short A/B nudge · hold A = save+exit · hold B = exit (no save)

Both steps show the same photo dithered to the 4 real levels, so you judge each gray against a real image rather than a swatch. Values persist to /data/display_calibration.json and re-apply at boot (EInk forgets waveform overrides on restart).

The screen leans on a grayscale rendering pipeline the badge didn't have yet, so this PR brings it along:

• Display switches from the old EInk.new/… handle + manual 1-bit bit-packing to the eink singleton draw API (EInk.draw(dither, opts)), which does its own dithering and understands mode: :grayscale.
• Screen now forwards a :render_opts assign into the draw call, so any screen can opt into mode: :grayscale the same way it sets :button_hints.
• DisplayMock gained a quantize4 path so the host simulator shows the 4 real levels instead of hard black/white.
• The EInk singleton is supervised (before Display) and configured via config :eink in config/target.exs.
• New: NameBadge.Calibration (the persistence + LUT logic) and NameBadge.Screen.Settings.Calibrate (the wizard UI), plus a settings-menu entry.

Notes for reviewers

• dither bumped ~> 0.1.1 → ~> 0.2.4 (override: true) — the new eink pipeline requires it.
• eink is pinned to a git commit, not a branch or hex version. That's deliberate and temporary (see the blocker above); the comment in mix.exs says what to swap it to.
• On host, eink isn't compiled (targets: [:trellis]), so EInk xref warnings during a host compile are expected — the simulator uses DisplayMock and never calls into EInk.