`saveSecretKey` unconditionally overwrites any existing secret with no guard

[ndii-dev]

secureStorage.ts's saveSecretKey always calls SecureStore.setItemAsync against the same fixed key with no existing-value check. Combined with the create/import screens both calling it, a user who navigates back to /auth/create after already onboarding (e.g. via a deep link, or a stale navigation stack after the missing-persistence bug in a related issue) can silently destroy access to their original address by generating a new one over it, with zero confirmation step.

Definition of done:

• saveSecretKey (or its callers) refuses to overwrite an existing secret without an explicit, scary, confirmed "replace wallet" step
• Covers both the create and import entry points
• Test asserting an existing secret survives an accidental re-entry into either screen

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This is not a starter-issue. The Definition of done above is the full
acceptance criteria, not a subset to sample from — a PR that addresses part
of it is an unfinished issue, not a smaller one. Your PR must include, in
the PR description itself:

• Root cause / design-decision rationale in your own words — not a restatement of this issue
• Every Definition of done bullet above addressed explicitly, with a one-line note on how
• Evidence the code actually runs: pasted test/build output, a screen recording or before/after screenshots for UI changes, or real (non-mocked) logs for network/contract-facing work
• New or updated tests included and shown passing (paste the output)
• Any adjacent/related behavior this issue calls out re-verified, not assumed unaffected

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[Ayinkx]

Hi i would like to work on this issue please assign to me
I can do it perfectly

[grantfox-oss]

🦊 GrantFox — @Ayinkx1 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #32)
2. Your PR will be reviewed by the Orbit-Wal maintainers

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