Add touch-friendly SeedQR transcription viewer

[schnuartz-ai]

Summary

Implements a guided, zone-based SeedQR transcription viewer, adapted from
SeedSigner's SeedQR transcription workflow to Specter DIY's 480x800
touchscreen (see SeedSigner's docs/seed_qr/README.md for the reference UX).

Selecting SeedQR (digits) or Compact SeedQR (binary) from the
mnemonic export menu now opens a dedicated overview screen: the complete
QR code, split into labeled sections, with a coordinate axis (row letters
/ column numbers) around it and a "Tap a section to enlarge" hint. Tapping
a section opens it hugely enlarged, with crisp square modules, visible
grid lines, and Left/Right/Up/Down navigation to neighboring sections —
making it practical to copy a SeedQR onto paper or a metal backup plate by
hand. Returning to the overview preserves the section you were last
looking at. Plaintext QR export is unchanged.

(I'll attach a few simulator screenshots to this description separately.)

Design

• src/seedqr.py: pure matrix/geometry helpers (matrix generation +
  validation, section geometry, touch-to-section mapping, navigation) with
  no lvgl dependency, so this logic is unit-testable on a desktop
  Python interpreter without a display or firmware build.
• src/gui/screens/seedqr.py: SeedQROverviewScreen + SeedQRZoomScreen,
  and the show_seedqr() overview<->zoom navigation loop. Both screens
  render the same canonical module matrix (generated once via
  seedqr.generate_matrix()), so the overview and every zoomed section
  can never disagree about module data.
• src/keystore/ram.py: routes the Standard/Compact SeedQR menu choices
  to show_seedqr(); the Plaintext QR path is unchanged except it now
  passes sensitive=True.
• Section sizing matches SeedSigner: a 21x21 QR groups into 7x7-module
  sections (3x3 grid); every larger SeedQR size groups into 5x5-module
  sections. A non-square-divisible size (e.g. 29x29) has a smaller
  trailing row/column, which is rendered at its true, smaller size (not
  padded out with filler modules).

Security

This handles wallet recovery material, so:

• The mnemonic, SeedQR digit string, Compact SeedQR bytes, and the QR
  module matrix are never printed, logged, or persisted.
• While reviewing gui/components/qrcode.py I found a pre-existing
  unconditional print(text) that logged the full QR payload even on
  real hardware, not just the simulator. Removed it, and added a
  sensitive flag (threaded through add_qrcode/QRAlert) so simulator
  debug logging can be suppressed per-QR; it's now set for every
  mnemonic-related QR, including the pre-existing Plaintext export path.
• Both new screens drop their references to the secret matrix on LVGL
  DELETE.

Companion fix (separate repo)

While building the matrix-generation helper I found a pre-existing bug in
the f469-disco submodule's qrcode usermod: _qrcode_encode() decided
text-vs-binary mode by scanning for an embedded 0x00 byte, so a bytes
payload with no zero byte (common for Compact SeedQR's raw entropy) fell
through to qrcodegen_encodeText(), which expects a NUL-terminated string
and could read past the intended data into uninitialized stack memory.
Fixed at diybitcoinhardware/f469-disco#43 (using the actual Python object
type instead of content-sniffing). I don't have write access to that
repo's upstream, so it's a separate PR; the f469-disco submodule pointer
in this PR is unchanged (still upstream's commit) — once #43 merges,
the submodule pointer here should be bumped in a follow-up commit.

Test plan

• python3 run_native_tests.py — 41/41 tests pass (encoding, section
  geometry, touch mapping, navigation, and print/log security
  regressions for the pure seedqr module).
• python3 -m compileall src test — clean.
• make unix (real firmware build, including this submodule fix) —
  builds successfully; confirmed the compiled qrcode.encode_to_string()
  produces the correct matrix size for all 4 SeedQR combinations
  (12/24-word x Standard/Compact), including entropy with no embedded
  zero byte.
• Manually exercised all 4 combinations in the unix simulator: full
  overview, tapping into first/middle/last/partial sections, all four
  nav directions, back-to-overview section
• Not tested on physical hardware.

This PR implements the SeedQR transcription mode requested in #212, including the sectioned overview and enlarged grid-based navigation for copying the QR to paper or metal.

Fixes #212

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