feat: update ssss to centeralise key logic

[ab-famedly]

Added reusable public migration APIs in SDK SSSS:

• analyzeEncryptedSecrets()
• orderedCandidateKeyIds(...)
• migrateSecretsToKey(...)

Also added private SDK helper:

• _tryOpenAndUnlockKey(...)

These now contain the logic to centralised (secret discovery, key iteration, migration, cache warm-up via maybeCacheAll()).

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[codecov]

Codecov Report

❌ Patch coverage is 38.88889% with 99 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.25%. Comparing base (2ba4922) to head (f89b314).

Files with missing lines	Patch %	Lines
lib/encryption/ssss.dart	31.20%	86 Missing ⚠️
lib/encryption/utils/bootstrap.dart	33.33%	6 Missing ⚠️
lib/src/utils/native_implementations.dart	80.76%	5 Missing ⚠️
...worker/native_implementations_web_worker_stub.dart	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2317      +/-   ##
==========================================
- Coverage   55.76%   55.25%   -0.51%     
==========================================
  Files         160      160              
  Lines       19850    19957     +107     
==========================================
- Hits        11069    11028      -41     
- Misses       8781     8929     +148     

Files with missing lines	Coverage Δ
...worker/native_implementations_web_worker_stub.dart	0.00% <0.00%> (ø)
lib/src/utils/native_implementations.dart	42.30% <80.76%> (+19.23%)	⬆️
lib/encryption/utils/bootstrap.dart	60.90% <33.33%> (+2.29%)	⬆️
lib/encryption/ssss.dart	59.50% <31.20%> (-25.50%)	⬇️

... and 3 files with indirect coverage changes

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[td-famedly]

eh why are we doing this? (also the previous comment was helpful so someone does not remove the getStored check)

[ab-famedly]

We need either “assign the mutated copy back into content” or “mutate the nested map in place,” because tryGetMap copies, stripping only the copy would upload unstripped account data. I've added the comment back though.

[td-famedly]

another review, looks like some of the methods are not really needed