feat(widget): Widget 配置、loader 与右下角悬浮入口

生成可嵌入站点的 loader 与悬浮按钮，支持展开收起和移动端；附独立宿主测试页。

backend/assets/widget.css

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+.new-chat-confirm[data-v-6c78f353]{display:flex;align-items:center;justify-content:space-between;gap:12px;flex-wrap:wrap;padding:10px 14px;border-bottom:1px solid var(--cm-hairline, rgba(0, 0, 0, .07));background:var(--cm-agent-bg, rgba(0, 0, 0, .03));font-family:var(--cm-body-font, inherit)}.new-chat-confirm__question[data-v-6c78f353]{margin:0;font-size:12.5px;line-height:1.35;color:var(--cm-text, inherit)}.new-chat-confirm__actions[data-v-6c78f353]{display:flex;gap:8px;margin-left:auto}.new-chat-confirm__button[data-v-6c78f353]{padding:5px 11px;border-radius:var(--cm-field-radius, 8px);border:1px solid var(--cm-border, rgba(0, 0, 0, .12));background:transparent;color:var(--cm-text, inherit);font-family:inherit;font-size:12px;font-weight:500;cursor:pointer;transition:opacity .15s ease}.new-chat-confirm__button--primary[data-v-6c78f353]{border-color:transparent;background:var(--cm-accent, #f34611);color:var(--cm-on-accent, #ffffff)}.new-chat-confirm__button[data-v-6c78f353]:disabled{opacity:.5;cursor:default}.new-chat-confirm__button[data-v-6c78f353]:not(:disabled):hover{opacity:.85}@media (pointer: coarse){.new-chat-confirm[data-v-6c78f353]{padding:10px 12px}.new-chat-confirm__button[data-v-6c78f353]{min-height:40px;padding:8px 14px}}@media (prefers-reduced-motion: reduce){.new-chat-confirm__button[data-v-6c78f353]{transition:none}}.askai[data-v-edea9534]{display:flex;flex-direction:column;height:100%;min-height:0;background:var(--cm-card);color:var(--cm-text);font-family:var(--cm-body-font, inherit)}.askai__bar[data-v-edea9534]{display:flex;align-items:center;gap:10px;padding:14px 14px 14px 16px;border-bottom:1px solid var(--cm-border);flex-shrink:0}.askai__bar-icon[data-v-edea9534]{color:var(--cm-accent);flex-shrink:0}.askai__input[data-v-edea9534]{flex:1;min-width:0;border:none;outline:none;background:transparent;color:var(--cm-text);font-size:16px;font-family:inherit;padding:2px 0}.askai__input[data-v-edea9534]::placeholder{color:var(--cm-muted)}.askai__input[data-v-edea9534]:disabled{opacity:.6}.askai__new[data-v-edea9534]{position:relative;display:inline-flex;align-items:center;justify-content:center;width:28px;height:26px;border:1px solid var(--cm-border);background:transparent;border-radius:6px;cursor:pointer;color:var(--cm-muted);font-family:inherit;flex-shrink:0;transition:color .15s ease,border-color .15s ease}.askai__new[data-v-edea9534]:hover:not(:disabled){color:var(--cm-text);border-color:var(--cm-accent)}.askai__new[data-v-edea9534]:disabled{opacity:.5;cursor:default}.askai__new--armed[data-v-edea9534]{color:var(--cm-text);border-color:var(--cm-accent)}.askai__close[data-v-edea9534]{border:1px solid var(--cm-border);background:transparent;border-radius:6px;padding:3px 7px;cursor:pointer;color:var(--cm-muted);flex-shrink:0;transition:color .15s ease,border-color .15s ease}.askai__close[data-v-edea9534]:hover{color:var(--cm-text);border-color:var(--cm-accent)}.askai__kbd[data-v-edea9534]{font-size:11px;font-weight:600;letter-spacing:.02em;font-family:inherit}.askai__body[data-v-edea9534]{flex:1;min-height:0;overflow-y:auto;scrollbar-gutter:stable;padding:18px 20px 22px}.askai__content[data-v-edea9534]{display:flow-root}.askai__intro[data-v-edea9534]{margin-bottom:18px}.askai__title[data-v-edea9534]{margin:0 0 4px;font-size:19px;font-weight:600;letter-spacing:-.01em}.askai__subtitle[data-v-edea9534]{margin:0;font-size:14px;line-height:1.5;color:var(--cm-muted)}.askai__label[data-v-edea9534]{display:block;margin:0 0 8px;font-size:11px;font-weight:600;text-transform:uppercase;letter-spacing:.07em;color:var(--cm-muted)}.askai__suggestions[data-v-edea9534]{display:flex;flex-direction:column;gap:6px}.askai__suggestion[data-v-edea9534]{display:flex;align-items:center;justify-content:space-between;gap:12px;width:100%;text-align:left;padding:11px 13px;border:1px solid var(--cm-border);border-radius:10px;background:var(--cm-agent-bg);color:var(--cm-text);font-size:14px;font-family:inherit;cursor:pointer;transition:border-color .15s ease,transform .15s ease}.askai__suggestion[data-v-edea9534]:hover:not(:disabled){border-color:var(--cm-accent);transform:translate(2px)}.askai__suggestion[data-v-edea9534]:disabled{opacity:.55;cursor:default}.askai__suggestion svg[data-v-edea9534]{color:var(--cm-muted);flex-shrink:0}.askai__turn+.askai__turn[data-v-edea9534]{margin-top:20px}.askai__question[data-v-edea9534]{margin:0 0 10px;font-size:16px;font-weight:600;line-height:1.4;letter-spacing:-.01em}.askai__answer[data-v-edea9534]{font-size:14.5px;line-height:1.65;color:var(--cm-text);overflow-wrap:break-word}.askai__answer--streaming[data-v-edea9534]>:last-child:after{content:"";display:inline-block;width:6px;height:1em;margin-left:2px;vertical-align:-.15em;background:var(--cm-accent, currentColor);animation:askai-blink-edea9534 1s steps(1) infinite}@keyframes askai-blink-edea9534{0%,50%{opacity:1}51%,to{opacity:0}}.askai__answer[data-v-edea9534] p{margin:0 0 10px}.askai__answer[data-v-edea9534] p:last-child{margiLine truncated