Add MarkdownEditor component, asset image upload, and rich description editing

Introduce MarkdownEditor built on @mdxeditor/editor with headings,
lists, links, quotes, image upload with drag-and-drop, and themed CSS
integration. Add asset image upload API (routes, service, storage) and
wire image upload into InlineEditor multiline mode, NewIssueDialog,
NewProjectDialog, GoalDetail, IssueDetail, and ProjectDetail
description fields. Tighten prompt template editor styling.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/index.css

@@ -191,3 +191,170 @@
     animation: none;
   }
 }
+
+/* MDXEditor theme integration */
+.paperclip-mdxeditor-scope,
+.paperclip-mdxeditor {
+  --baseBase: var(--background);
+  --baseBg: transparent;
+  --baseBgSubtle: color-mix(in oklab, var(--accent) 35%, transparent);
+  --baseLine: var(--border);
+  --baseSolid: var(--muted-foreground);
+  --baseSolidHover: var(--foreground);
+  --baseText: var(--muted-foreground);
+  --baseBorderColor: var(--border);
+  --baseBorder: var(--border);
+  --baseBorderHover: var(--ring);
+  --baseTextContrast: var(--foreground);
+  --baseTextContrastMuted: var(--muted-foreground);
+  --baseTextEmphasis: var(--foreground);
+  --basePageBg: var(--background);
+  --baseRadius: var(--radius);
+  --baseLineHeight: 1.5;
+  --accentBorder: color-mix(in oklab, var(--primary) 35%, var(--border));
+  --accentSolid: var(--primary);
+  --accentSolidHover: var(--primary);
+  --accentLine: color-mix(in oklab, var(--primary) 20%, transparent);
+  --accentBg: var(--accent);
+  --accentBgHover: color-mix(in oklab, var(--accent) 80%, var(--background));
+  --accentBgActive: color-mix(in oklab, var(--accent) 72%, var(--background));
+  --accentText: var(--accent-foreground);
+  font-family: inherit;
+  font-size: 0.875rem;
+  line-height: 1.5;
+  color: var(--foreground);
+}
+
+.paperclip-mdxeditor-scope [class*="_iconButton_"],
+.paperclip-mdxeditor [class*="_iconButton_"] {
+  color: var(--baseText);
+}
+
+.paperclip-mdxeditor-scope [class*="_iconButton_"]:hover,
+.paperclip-mdxeditor [class*="_iconButton_"]:hover {
+  color: var(--baseTextContrast);
+}
+
+.paperclip-mdxeditor .mdxeditor-root-contenteditable {
+  min-height: 2.5rem;
+  padding: 0.375rem 0.625rem;
+  line-height: 1.5;
+}
+
+.paperclip-mdxeditor--borderless .mdxeditor-root-contenteditable {
+  padding: 0.2rem 0;
+}
+
+.paperclip-mdxeditor [class*="_placeholder_"] {
+  padding: 0.375rem 0.625rem;
+  font-family: inherit;
+  font-size: 0.875rem;
+  line-height: 1.5;
+  color: var(--muted-foreground);
+}
+
+.paperclip-mdxeditor--borderless [class*="_placeholder_"] {
+  padding: 0.2rem 0;
+}
+
+.paperclip-mdxeditor-content {
+  font-family: inherit;
+  font-size: inherit;
+  line-height: inherit;
+  color: inherit;
+}
+
+.paperclip-mdxeditor-content > *:first-child {
+  margin-top: 0;
+}
+
+.paperclip-mdxeditor-content > *:last-child {
+  margin-bottom: 0;
+}
+
+.paperclip-mdxeditor-content p {
+  margin: 0;
+}
+
+.paperclip-mdxeditor-content p + p {
+  margin-top: 0.4rem;
+}
+
+.paperclip-mdxeditor-content ul,
+.paperclip-mdxeditor-content ol {
+  margin: 0.35rem 0;
+  padding-left: 1.1rem;
+}
+
+.paperclip-mdxeditor-content ul {
+  list-style: disc;
+}
+
+.paperclip-mdxeditor-content ol {
+  list-style: decimal;
+}
+
+.paperclip-mdxeditor-content li {
+  display: list-item;
+  margin: 0.15rem 0;
+}
+
+.paperclip-mdxeditor-content li::marker {
+  color: var(--muted-foreground);
+}
+
+.paperclip-mdxeditor-content h1,
+.paperclip-mdxeditor-content h2,
+.paperclip-mdxeditor-content h3 {
+  margin: 0.4rem 0 0.25rem;
+  line-height: 1.3;
+}
+
+.paperclip-mdxeditor-content img {
+  max-height: 18rem;
+  border-radius: calc(var(--radius) - 2px);
+}
+
+.paperclip-mdxeditor-content blockquote {
+  margin: 0.45rem 0;
+  padding-left: 0.7rem;
+  border-left: 2px solid var(--border);
+  color: var(--muted-foreground);
+}
+
+.paperclip-mdxeditor-content code {
+  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
+  font-size: 0.84em;
+}
+
+.paperclip-mdxeditor-content pre {
+  margin: 0.5rem 0;
+  padding: 0.45rem 0.55rem;
+  border: 1px solid var(--border);
+  border-radius: calc(var(--radius) - 3px);
+  background: color-mix(in oklab, var(--accent) 50%, transparent);
+  overflow-x: auto;
+}
+
+/* Keep MDXEditor popups above app dialogs when editor is inside a modal. */
+.paperclip-mdxeditor-scope [class*="_dialogOverlay_"],
+.paperclip-mdxeditor [class*="_dialogOverlay_"] {
+  z-index: 80;
+}
+
+.paperclip-mdxeditor-scope [class*="_dialogContent_"],
+.paperclip-mdxeditor-scope [class*="_largeDialogContent_"],
+.paperclip-mdxeditor-scope [class*="_popoverContent_"],
+.paperclip-mdxeditor-scope [class*="_linkDialogPopoverContent_"],
+.paperclip-mdxeditor-scope [class*="_tableColumnEditorPopoverContent_"],
+.paperclip-mdxeditor-scope [class*="_toolbarButtonDropdownContainer_"],
+.paperclip-mdxeditor-scope [class*="_toolbarNodeKindSelectContainer_"],
+.paperclip-mdxeditor [class*="_dialogContent_"],
+.paperclip-mdxeditor [class*="_largeDialogContent_"],
+.paperclip-mdxeditor [class*="_popoverContent_"],
+.paperclip-mdxeditor [class*="_linkDialogPopoverContent_"],
+.paperclip-mdxeditor [class*="_tableColumnEditorPopoverContent_"],
+.paperclip-mdxeditor [class*="_toolbarButtonDropdownContainer_"],
+.paperclip-mdxeditor [class*="_toolbarNodeKindSelectContainer_"] {
+  z-index: 81 !important;
+}