Separate single-bucket entity from accounts and unify add flow

[imggion]

Motivation

• Single-bucket entries are a different domain from full accounts and should be persisted and modeled separately to avoid mixing concerns.
• The add/connection UX should present a single entry point where the user picks whether to create an Account or a Single Bucket before entering credentials.

Description

• Introduced a dedicated type and DAO for single-bucket entries: SingleBucketConfig and SingleBucketDao, and added a Flyway migration V2__add_single_bucket_support.sql to create the single_buckets table.
• Restored AccountConfig/AccountDao to be account-only and removed the previous singleBucketName field from the account persistence model.
• Replaced the two separate add dialogs with a single AccountDialog that has an EntryType selector (ACCOUNT / SINGLE_BUCKET) and returns a ConnectionInput; deleted the old SingleBucketDialog.
• Updated MainWindow to handle two distinct flows (persist/load/connect/delete) for AccountConfig vs SingleBucketConfig, to create sessions for single-bucket entries, and to normalize/validate inputs with new helpers (normalizeSingleBucket, toSessionAccountConfig, validateConnectionInput).
• Updated the sidebar to represent single-bucket entries as leaf nodes (no expand/collapse) and added addSingleBucket / updateSingleBucket methods and an AccountNode variant that carries a singleBucket flag and bucketName.
• Small UI/menu changes to expose a single empty-area action Add Connection and to disable the Create bucket action for single-bucket entries via AccountContextMenu.

Testing

• Ran ./gradlew test which failed in this environment because publishing.gradle expects the giteaToken property and the build evaluation aborts.
• Ran ./gradlew test -PgiteaToken=dummy which failed during dependency resolution due to HTTP 403 from Maven Central in this environment so compilation/tests could not complete.
• Ran ./gradlew compileJava -PgiteaToken=dummy --offline which failed because no dependency cache is available offline, so compilation could not be validated locally.

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Codex Task