feat: inline editing of node values from the properties panel

[shaunpatterson]

What

Lets users edit graph data directly from the node properties panel instead of hand-writing mutations — the headline capability of commercial graph IDEs like G.V():

• Each scalar attribute row gets edit and delete actions: edit opens an inline input (number input for numbers, true/false select for booleans), Enter/✓ saves, Escape cancels. Delete is two-step (trash → "sure?").
• + Add value sets a new predicate on the node.
• Server errors surface inline in the panel; successful edits update the in-memory node and show a transient ✓ saved.

Safety

lib/mutations.js builds single-triple N-Quad mutations (commitNow) through the existing dgraph client, with:

• No N-Quad injection: uids must match 0x[hex], predicate names reject angle brackets/whitespace/quotes; string literals are escaped (backslashes, quotes, newlines, tabs).
• No silent retyping: values are written with the original value's type (xs:int / xs:float / xs:boolean, plain strings), and user input is coerced back to that type with a clear error when it doesn't parse.

Testing

• 11 unit tests on the mutation builders (escaping, injection rejection, typed literals, delete-one vs delete-all, coercion).
• Every generated mutation shape verified against a live Dgraph v25: set string with escaped quotes, set typed int, delete specific value, delete all values — all accepted, final state confirmed by query.
• npm run build passes.

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