feat(web): add customizable input placeholder for Agent/Chatflow/Chatbot web app

[CyberPU2099]

Summary

Adds an Input placeholder field to Web App Settings that lets owners customize the chat input box placeholder text. Available only for chat-style apps (Chatbot / Agent / Chatflow) and gated behind the same paid-plan entitlement (webappCopyrightEnabled) as Copyright.

• Free plan: Upgrade pill + read-only mini-chat-input preview + hover tooltip
• Paid plan: editable up to 64 chars with live N / 64 counter; the field renders as a mini chat input box (text + disabled send button) and on blur swaps the entered value to placeholder-gray so owners see the actual rendered effect
• Runtime: takes effect in both standalone Web App and embedded chatbot; empty / whitespace-only falls back to the existing Talk to {{botName}} i18n string
• i18n: en-US, zh-Hans, ja-JP (other locales intentionally deferred to translation flow per repo convention)

Scope

Frontend only (web/). No new dependencies. 8 files, +106/-2.

File	Change
web/types/app.ts	SiteConfig.input_placeholder: string
web/models/share.ts	SiteInfo.input_placeholder?: string
web/i18n/{en-US,zh-Hans,ja-JP}/app-overview.json	4 keys × 3 locales
web/app/components/app/overview/settings/index.tsx	New settings row, gated by mode + plan
web/app/components/base/chat/chat/index.tsx	Pass customPlaceholder to ChatInputArea
web/app/components/base/chat/chat/chat-input-area/index.tsx	Accept customPlaceholder prop and prefer it over the default i18n string when not readonly

⚠️ Backend dependency (BLOCKING)

This PR ships the frontend only. End-to-end functionality requires a backend counterpart:

1. DB migration — add input_placeholder VARCHAR(64) NOT NULL DEFAULT '' to the sites table.
2. Site GET (console + share/web API) — include input_placeholder in the serialized site payload so the frontend can read what was saved.
3. Site PATCH — accept the field in the request body, enforce len <= 64, persist.
4. Entitlement — confirm whether to reuse webapp_copyright_enabled (current frontend gate) or add a dedicated webapp_input_placeholder_enabled flag. If the backend splits the gate, swap a single constant in settings/index.tsx.

Without these, the field renders correctly in Settings but the value is silently dropped on save (backend ignores the unknown field, GET returns no value, runtime falls back to default).

Test plan

• Chatbot app → Settings → "Show more settings" shows the Input placeholder row above Copyright
• Agent app → shows the row
• Chatflow app → shows the row
• Workflow app → row is hidden
• Text Generator app → row is hidden
• Free-plan workspace (Cloud edition, sandbox): Upgrade pill visible, input disabled, hover wrapper → tooltip "Upgrade to customize the placeholder" (zh: 升级后可自定义提示语 / ja: アップグレードするとプレースホルダーをカスタマイズできます), no character counter
• Paid-plan workspace: input editable, N / 64 counter, maxLength enforced; focus → text in primary color, blur with value → text switches to placeholder-gray (live preview)
• Language switch (Settings → Language → en/zh/ja) — label, tip, tooltip, placeholder all match canonical translations
• After backend lands: chat input box in standalone Web App picks up the configured placeholder; empty falls back to Talk to {botName}
• After backend lands: embedded chatbot iframe shows the configured placeholder
• readonly chat (e.g. shared preview without permission) still uses chat.inputDisabledPlaceholder rather than the custom string
• Empty / whitespace-only string saves as empty and falls back to default at runtime

Related

• PRD:
• Prototype reference: local HTML mockup matched to this implementation

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.79%. Comparing base (a3309cd) to head (cce3410).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
web/app/components/app/overview/settings/index.tsx	85.71%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #37790      +/-   ##
==========================================
- Coverage   85.79%   85.79%   -0.01%     
==========================================
  Files        5001     5001              
  Lines      255153   255181      +28     
  Branches    48288    48307      +19     
==========================================
+ Hits       218911   218932      +21     
- Misses      32048    32055       +7     
  Partials     4194     4194              

Flag	Coverage Δ
dify-ui	94.94% <ø> (ø)
web	85.44% <86.66%> (-0.01%)	⬇️

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