fix: Persist passphrase across service worker restarts

[macterra]

Summary

• Move passphrase and extension state from in-memory variables to chrome.storage.session
• Survives MV3 service worker termination (no more re-login on every archon:// link click)
• Session storage clears on browser close, maintaining same security model
• Also works in Firefox (MV2) via browser.storage.session (Firefox 115+)

Closes #221

Test plan

• Login to Chrome extension, wait for service worker to stop, click an archon://auth link — should not require re-login
• Close and reopen browser — should require login again
• Verify Nostr origin approvals still work
• Test in Firefox extension

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Persists the browser extension’s passphrase and general “extension state” across MV3 service worker restarts by moving them from in-memory globals into chrome.storage.session, aligning their lifetime with the browser session (clears on browser close).

Changes:

• Replace in-memory passphrase with getPassphrase/setPassphrase backed by chrome.storage.session.
• Replace in-memory extensionState object with getExtensionState/setExtensionState backed by chrome.storage.session.
• Update chrome.runtime.onMessage handlers to read/write passphrase/state asynchronously via session storage.

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