Light refactor of Electrum seed support

[kdmukai]

Description

Light refactor of #513:

• A View should only ever invoke a single Screen. Pulled the Electrum seed warning screen into its own View. Also gain some modest DRY efficiency and easier future maintenance this way.
• The SettingsEntry for the Electrum seed option used a display_name value that was way too long to fit in the button list (Settings -> Advanced) that would leave a rendering artifact on the far right edge (scroll past the Electrum option to the bottom of the list).
• Slightly tightens up ElectrumSeed mnemonic validation.

Housekeeping, more robust testing:

• Adds a screenshot for the Electrum seed warning.
• Adds a screenshot for the "Advanced" section of the Settings list.
• Adds flow tests:
  • Basic Electrum mnemonic entry.
  • Export xpub should respect ElectrumSeed.override_script.
  • Load PSBT side flow to load an Electrum mnemonic and return to PSBT flow (incomplete)
  • Address Explorer side flow to load an Electrum mnemonic and return.
• Additional tests for ElectrumSeed:
  • Check mnemonic length requirement
  • Reject most BIP-39 mnemonics but confirm that some are valid for both BIP-39 and Electrum.

Reversions:

• Reverts the changes to SeedQREncoder that were probably added at an earlier phase when Electrum seed + SeedQR was being considered. Downstream effect of this reversion includes seed_views.py, test_encodepsbtqr.py, test_seedqr.py.

Misc:

• Remove bool return from Seed._generate_seed as it is never used.
• Naming convention conformance in Seed: wallet_type -> sig_type; the term "wallet" is just too overused and poorly specified in bitcoin. Note: embit_utils does still use wallet_type but in order to keep this ever-larger PR in check and since it's older, existing code, I left that refactor for another day.

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Settings list issue before the fix in this PR:

After:

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New/Updated screenshots:

After moving part of the long settings name into help_text:

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This pull request is categorized as a:

• [x] Code refactor
• [x] Additional tests

Checklist

• [x] I’ve run pytest and made sure all unit tests pass before sumbitting the PR

If you modified or added functionality/workflow, did you add new unit tests?

• [x] Yes

I have tested this PR on the following platforms/os:

• [x] Raspberry Pi OS Manual Build
• [ ] SeedSigner OS on a Pi0/Pi0W board
• [ ] Other

Note: Keep your changes limited in scope; if you uncover other issues or improvements along the way, ideally submit those as a separate PR. The more complicated the PR the harder to review, test, and merge.

[jdlcdl]

As of f06666eb

ACK tested

Everything is working for me:

• electrum 12w seed with custom extension
• exporting xpub to sparrow
• address explorer and address verification matching
• psbt reading/signing (of course).

[kdmukai]

• electrum 12w seed with custom extension

Ooh, I should add the "custom extension" wrinkle to the FlowTest.

[jdlcdl]

I've retested this w/ a modified version on top of 6d114f7:

• pulled in #573 on top of this
• resolved the 2 conflicts in seed test flows: (importing FlowTestUnexpectedRedirectException and FlowTestInvalidButtonDataSelectionException from base test; then taking the 573 version of the conflicting test).

w/ that said, I repeat my ACK tested from earlier today w/ same manual tests performed.

[newtonick]

tACK