feat: free trial usage limit banner

[4pmtong]

Related Issue

Closes #

Description

Adds free trial and credits usage guidance across the chat input and Eigent Cloud model settings.

• Adds a usage limit banner to the chat input for trial daily limit, trial total limit, free credits, and monthly credits.
• Polls /api/v1/subscription and /api/v1/user/current_credits to keep usage state fresh.
• Shows warning/exhausted states when usage reaches configured thresholds.
• Handles /api/v1/user/key returning code=22 as a cloud usage limit state instead of a missing model configuration.
• Disables chat input only when Eigent Cloud is selected and the cloud trial limit is reached.
• Allows local/custom models to work after switching away from Eigent Cloud, as long as a provider is configured.
• Updates the Eigent Cloud settings card to show current credits, trial-specific copy, and an upgrade CTA.
• Adds an upgrade confirmation dialog for trial users.
• Adds i18n strings for chat usage limits and trial upgrade UI across supported locales.

Testing Evidence (REQUIRED)

• I have included human-verified testing evidence in this PR.
• This PR includes frontend/UI changes, and I attached screenshot(s) or screen recording(s).
• No frontend/UI changes in this PR.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Contribution Guidelines Acknowledgement

• I have read and agree to the Eigent Contribution Guideline