Client-side encrypted search (replaces server search)

[codebestia]

Description:
Since the server can no longer search ciphertext (#124), implement local search over the decrypted message cache (#185): a local inverted index or substring search in IndexedDB/WebWorker. Scope to what the device has decrypted. Document the UX tradeoff (only searches synced history).

Acceptance criteria:

• Local search returns relevant decrypted messages
• Runs off the main thread for large histories
• Clearly scoped to locally available messages

[vaishnavidesai09]

@vaishnavidesai09 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hey maintainer give me this issue!!.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @vaishnavidesai09 to this issue.

[Caneryy]

@Caneryy has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, can I work on this issue?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Caneryy to this issue.