Add self-hosted instance login support

[JeremyZeng77]

Fixes #12

Summary

• add a persistent Stoat/Revolt instance API setting on the login screen
• restore the saved instance before startup login/session checks
• make API, websocket, Autumn, January, web app, and invite endpoints resolve from the selected instance
• fetch the Revolt API root metadata to use server-provided ws, features.autumn.url, and features.january.url when available, with /autumn, /january, and /ws fallbacks for typical self-hosted deployments

Verification

• git diff --check
• Attempted ./gradlew.bat :app:compileDebugKotlin --console=plain; local environment is missing Android SDK / ANDROID_HOME, so Android compilation could not run here.

Bounty payout

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[JeremyZeng77]

CI note: the current track_pr failure appears to be in the repository workflow setup, not in this Android patch. The job exits while running gh api graphql because GH_TOKEN is not set in the workflow environment:

gh: To use GitHub CLI in a GitHub Actions workflow, set the GH_TOKEN environment variable.
Process completed with exit code 4.

The DCO check passed. The PR body still has my available local verification details; I could not run Android compilation locally because this machine does not have Android SDK / ANDROID_HOME configured.

[JeremyZeng77]

CI note: the failing check is track_pr (workflow: Add PR to Board, event pull_request_target). It fails at step Get project data: https://github.com/stoatchat/for-android/actions/runs/25957799324/job/76307608126

This looks like a repo/project/permissions configuration issue (project id/owner, token permissions for Projects v2, etc.) rather than code in this PR.

[JeremyZeng77]

CI is currently failing due to the rack_pr workflow.

Details: the rack_pr job fails at step Get project data (the run exits almost immediately), and then later steps are skipped.
Run/job: https://github.com/stoatchat/for-android/actions/runs/25957799324/job/76307608126

This looks like a repo-side GitHub Projects/token permission or configuration issue (not a code/test failure in the PR). If you want, I can rebase/adjust once the workflow is unblocked.