feat(ui): match Scanner screen to Figma node 13:2

[ilmoniemi]

User Story

As a Pyrycode mobile user pairing for the first time, I want the Scanner screen to match the locked Figma design with full atmospheric polish, so the pairing moment reads as a premium developer-tool experience rather than a placeholder box.

Context

Phase 1's #12 shipped a stub Scanner — a dark rounded box with a single horizontal line and "Tap to pair" text. The Figma file's Scanner node 13:2 carries: dual radial gradients on the viewport (cool-blue 30/40, soft-coral 70/70), a scan-line with Schemes/Primary 85% opacity + DROP_SHADOW glow, 105 horizontal atmospheric stripes (1px tall, 4% white, every 7px), and a pyry pair mono+coral chip below the viewport.

Phase 1.5 visual-fidelity catchup. Follows the pattern proven by #57 (Welcome screen).

Acceptance Criteria

• Camera viewport renders with both radial gradients via Compose Brush.radialGradient (cool-blue at one corner, soft-coral at the other)
• Scan line + glow rendered (Schemes/Primary 85% alpha, drop-shadow blur)
• Atmospheric stripe overlay rendered (~105 stripes, low alpha — wrap as a single Canvas to avoid 105 nested composables)
• pyry pair chip rendered with mono font + coral accent, below viewport
• Zero hardcoded color / typography literals — all tokens via MaterialTheme.* (gradient stops are RGBA-frozen at creation per Figma workflow lessons; that constraint is acceptable)
• Tap-anywhere behavior preserved (fake-pair: writes pairedServerExists=true, navigates to ChannelList) — do NOT change MainActivity.kt's wiring

Technical Notes

• Existing scaffold: app/src/main/java/de/pyryco/mobile/ui/onboarding/ScannerScreen.kt (~96 LOC; rewrite per architect's spec)
• Gradient ellipse formula in the Design — Conversations Model doc is for Figma's set_fills; Compose uses Brush.radialGradient(center, radius) directly

Size Estimate

S — single file, ~120-150 production LOC. High visual density but bounded scope (no data-layer, no navigation changes).

Figma

https://www.figma.com/design/g2HIq2UyPhslEoHRokQmHG?node-id=13-2

[ilmoniemi]

🤖 Product Owner — creates structured issues

po agent has completed work on this ticket.

Agent output (click to expand)

Refinement complete. Body left unchanged (already in target shape), `size:s` applied, no `security-sensitive` (fake-pair UI, not internet-exposed). Dispatcher will auto-add `ready:po` and advance to In Architecture.

Ready for human review. Move to the next column when approved.

[ilmoniemi]

🤖 System Architect — defines Go interfaces, data flows, concurrency patterns

architect agent has completed work on this ticket.

Agent output (click to expand)

Spec written and committed at `docs/specs/architecture/60-scanner-screen-figma-polish.md`.

**Size:** S confirmed — single-file rewrite (~120–150 LOC), no edit fan-out (MainActivity wiring frozen by AC6), no overlapping in-flight branches.

**Design highlights:**
- Layered `Box` viewport: two `Brush.radialGradient` overlays (blue 30/40, coral 70/70) using `primary/tertiary` tokens with frozen alpha, single `Canvas` stripe loop terminating against measured height (avoids the 105-composable trap), 248dp reticle with 8 corner stubs + blurred scan-line glow via `Modifier.blur(12.dp)`.
- `pyry pair` mono+coral accent rendered as an `AnnotatedString` span (`FontFamily.Monospace` + `colorScheme.tertiary`) inside the hint card, not as a sibling `Text`.
- AC6 reconciliation: top-app-bar back IconButton and "Trouble scanning?" TextButton both fire `onTap` — flagged as the load-bearing Phase 1.5 contradiction with a one-line WHY comment in the developer's deliverable.
- Zero new files, zero new dependencies, zero MainActivity changes.

Ready for human review. Move to the next column when approved.