fix(lang): correct Ja key casing in connections locale

[49EHyeon42]

PR Checklist

What is the current behavior?

In src/lang/connections.ts, five keys define their Japanese translation
under a capitalized Ja: property instead of ja::

• topicTips
• enabledAutoResub
• disabledAutoResub
• enabledAutoResubDesc
• disabledAutoResubDesc

formati18n (src/utils/i18n.ts) looks up values[key][lang] using the
lowercase codes from supportLang, so these five resolve to undefined and
vue-i18n falls back to English.

Reproduced on MQTTX v1.13.0 with the UI language set to 日本語: in the
subscription dialog (サブスクリプション追加), the auto-resubscribe popover
shows "Auto resubscription is enabled / When enabled, MQTTX will
automatically resubscribe..." in English, while everything around it is
Japanese. Expected: 「自動再購読が有効」 etc.

The casing slip was introduced in 1be2461 together with these keys.

Issue Number

None

What is the new behavior?

The five properties are renamed to lowercase ja:. The Japanese
translations (already present in the file, just unreachable) now load, and
the popover and multi-topic tip display in Japanese.

Does this PR introduce a breaking change?

• Yes
• No

Casing-only change on five lines; no translation content is modified.

Specific Instructions

To verify: switch Settings → Language to 日本語, connect to any broker, open
the subscription dialog via サブスクリプション追加, and hover the info icon
next to the dialog title — the popover title/body should now be Japanese.

Other information

None