Replace floating warning/error toasts with a collapsible error collection panel in the web UI

Problem
The web UI currently displays warnings and errors as individual floating toasts
that auto-dismiss after 6-12 seconds. When multiple errors occur (e.g. during a
network interruption or provider error), they stack up and each one demands the
user's attention before disappearing. There is no way to review past errors
after they auto-dismiss, and no way to clear all errors at once.

Proposed solution
Replace the floating toast system with a collapsible error collection panel:

• A compact badge in the bottom-right corner showing the error count
• Clicking the badge expands a panel listing all errors with expandable diagnostics
• "Clear all" button to dismiss all errors
• "Refresh page" button for connection-related issues
• No auto-dismiss timers — the user controls when to clear

Why this fits Kimi Code
The existing toast system is noisy during error bursts and provides no way
to review past messages. A collection panel is the approach the TUI already
uses for warnings, and this brings the web UI in line with that pattern.

Scope

• apps/kimi-web/src/components/WarningToasts.vue — complete rewrite
• apps/kimi-web/src/i18n/locales/*/warnings.ts — new i18n keys

Start with apps/kimi-web/src/components/WarningToasts.vue to understand the current toast flow, then inspect the warning locale files under apps/kimi-web/src/i18n/locales/*/warnings.ts. Done means the web UI has a collapsible error-count panel with expandable diagnostics, Clear all and connection-related Refresh page actions, translated across the affected locale files, and no auto-dismiss timers.