fix(ui): parse nested frontmatter maps and arrays of maps (#1548)

* fix(ui): parse nested frontmatter maps and arrays of maps

extractFrontmatter hoisted indented child keys to the root, clobbering same-named keys and rendering wrong values.

Parse nested maps and arrays of maps; flat frontmatter renders byte-identically.

Closes #1485.

* fix(ui): treat a quoted frontmatter key as a mapping entry

A line starting and ending with a quote was rejected wholesale, so
`"title": "Doc"` parsed to {} and the row disappeared from the
frontmatter card. Accept a quoted KEY followed by `:` (plus the same
space-after-colon rule the unquoted branch applies); a line that is only
a quoted scalar is still not an entry.

Also record an array item's pending key at the key's own column
(lineIndent + 2) rather than the dash's, so a nested map used as an
item's first key no longer swallows the item's later sibling keys.

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Co-authored-by: Michael Ramos <support@plannotator.ai>